diff --git a/editor/AsyncSpellCheckTestHelper.jsm b/editor/AsyncSpellCheckTestHelper.jsm
index 6c28c4fafa46..77bfee17b162 100644
--- a/editor/AsyncSpellCheckTestHelper.jsm
+++ b/editor/AsyncSpellCheckTestHelper.jsm
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-var EXPORTED_SYMBOLS = ["onSpellCheck"];
+var EXPORTED_SYMBOLS = ["maybeOnSpellCheck", "onSpellCheck"];
 
 const SPELL_CHECK_ENDED_TOPIC = "inlineSpellChecker-spellCheck-ended";
 const SPELL_CHECK_STARTED_TOPIC = "inlineSpellChecker-spellCheck-started";
@@ -24,7 +24,7 @@ const SPELL_CHECK_STARTED_TOPIC = "inlineSpellChecker-spellCheck-started";
  *                         of the event loop have passed to determine it has not
  *                         started.
  */
-function onSpellCheck(editableElement, callback) {
+function maybeOnSpellCheck(editableElement, callback) {
   let editor = editableElement.editor;
   if (!editor) {
     let win = editableElement.ownerGlobal;
@@ -72,9 +72,9 @@ function onSpellCheck(editableElement, callback) {
   let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
   timer.init(
     function tick() {
-      // Wait an arbitrarily large number -- 50 -- turns of the event loop before
+      // Wait an arbitrarily large number -- 100 -- turns of the event loop before
       // declaring that no spell checks will start.
-      if (waitingForEnded || ++count < 50) {
+      if (waitingForEnded || ++count < 100) {
         return;
       }
       timer.cancel();
@@ -86,3 +86,22 @@ function onSpellCheck(editableElement, callback) {
     Ci.nsITimer.TYPE_REPEATING_SLACK
   );
 }
+
+/**
+ * Waits until spell checking has stopped on the given element.
+ *
+ * @param editableElement  The element being spell checked.
+ * @param callback         Called when spell check has completed or enough turns
+ *                         of the event loop have passed to determine it has not
+ *                         started.
+ */
+function onSpellCheck(editableElement, callback) {
+  const { TestUtils } = ChromeUtils.import(
+    "resource://testing-common/TestUtils.jsm"
+  );
+
+  let editor = editableElement.editor;
+  TestUtils.topicObserved(SPELL_CHECK_ENDED_TOPIC, s => s == editor).then(
+    callback
+  );
+}
diff --git a/editor/libeditor/crashtests/1444630.html b/editor/libeditor/crashtests/1444630.html
index 4c86cf39a66a..a4dc23294cf4 100644
--- a/editor/libeditor/crashtests/1444630.html
+++ b/editor/libeditor/crashtests/1444630.html
@@ -8,7 +8,7 @@ function load()
   textarea.focus();
 
   SpecialPowers.Cu.import("resource://reftest/AsyncSpellCheckTestHelper.jsm")
-  .onSpellCheck(textarea, () => {
+  .maybeOnSpellCheck(textarea, () => {
     let isc = SpecialPowers.wrap(textarea).editor.getInlineSpellChecker(false);
     let sc = isc.spellChecker;
 
diff --git a/editor/libeditor/tests/test_undo_after_spellchecker_replaces_word.html b/editor/libeditor/tests/test_undo_after_spellchecker_replaces_word.html
index abfa2d1e9d9f..789368758cfd 100644
--- a/editor/libeditor/tests/test_undo_after_spellchecker_replaces_word.html
+++ b/editor/libeditor/tests/test_undo_after_spellchecker_replaces_word.html
@@ -95,7 +95,7 @@ SimpleTest.waitForFocus(async () => {
 
       SpecialPowers.Cu.import(
         "resource://testing-common/AsyncSpellCheckTestHelper.jsm")
-      .onSpellCheck(aElement, () => {
+      .maybeOnSpellCheck(aElement, () => {
         SimpleTest.executeSoon(() => {
           aElement.addEventListener("beforeinput", onBeforeInput);
           aElement.addEventListener("input", onInput);
diff --git a/editor/spellchecker/tests/test_async_UpdateCurrentDictionary.html b/editor/spellchecker/tests/test_async_UpdateCurrentDictionary.html
index f95e35356988..24c38aae7c84 100644
--- a/editor/spellchecker/tests/test_async_UpdateCurrentDictionary.html
+++ b/editor/spellchecker/tests/test_async_UpdateCurrentDictionary.html
@@ -30,44 +30,29 @@ function start() {
     var isc = SpecialPowers.wrap(textarea).editor.getInlineSpellChecker(false);
     ok(isc, "Inline spell checker should exist after focus and spell check");
     var sc = isc.spellChecker;
-    isnot(sc.GetCurrentDictionary(), lang,
-          "Current dictionary should not be set yet.");
 
-    // First, set the lang attribute on the textarea, call Update, and make
-    // sure the spell checker's language was updated appropriately.
-    var lang = "en-US";
-    textarea.setAttribute("lang", lang);
-    sc.UpdateCurrentDictionary(function() {
-      is(sc.GetCurrentDictionary(), lang,
-         "UpdateCurrentDictionary should set the current dictionary.");
+    sc.setCurrentDictionaries(["testing-XX"]).then(() => {
+      is(true, false, "Setting a non-existent dictionary should fail");
+    }, () => {
+      let currentDictionaries = sc.getCurrentDictionaries();
 
-      // Second, make some Update calls, but then do a Set.  The Set should
-      // effectively cancel the Updates, but the Updates' callbacks should be
-      // called nonetheless.
-      var numCalls = 3;
-      for (var i = 0; i < numCalls; i++) {
-        sc.UpdateCurrentDictionary(function() {
-          is(sc.GetCurrentDictionary(), "",
-             "No dictionary should be active after Update.");
-          if (--numCalls == 0) {
-            // This will clear the content preferences and reset "spellchecker.dictionary".
-            sc.SetCurrentDictionary("");
-            SimpleTest.finish();
-          }
+      is(currentDictionaries.length, 0, "expected no dictionaries");
+      // First, set the lang attribute on the textarea, call Update, and make
+      // sure the spell checker's language was updated appropriately.
+      var lang = "en-US";
+      textarea.setAttribute("lang", lang);
+      sc.UpdateCurrentDictionary(function() {
+        currentDictionaries = sc.getCurrentDictionaries();
+        is(currentDictionaries.length, 1, "expected one dictionary");
+        is(sc.getCurrentDictionaries()[0], lang,
+          "UpdateCurrentDictionary should set the current dictionary.");
+        sc.setCurrentDictionaries([]).then(() => {
+          SimpleTest.finish();
         });
-      }
-      try {
-        sc.SetCurrentDictionary("testing-XX");
-      } catch (err) {
-        // Set throws NS_ERROR_NOT_AVAILABLE because "testing-XX" isn't really
-        // an available dictionary.
-      }
-      is(sc.GetCurrentDictionary(), "",
-         "No dictionary should be active after Set.");
+      });
     });
   });
 }
-
 </script>
 </pre>
 </body>
diff --git a/editor/spellchecker/tests/test_bug1100966.html b/editor/spellchecker/tests/test_bug1100966.html
index 4336bf38630f..a2335696f9cd 100644
--- a/editor/spellchecker/tests/test_bug1100966.html
+++ b/editor/spellchecker/tests/test_bug1100966.html
@@ -23,7 +23,7 @@ fivee sixx<br>
 
 <script class="testbody" type="application/javascript">
 
-let {onSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
+let {maybeOnSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
 
 /** Test for Bug 1100966 **/
 SimpleTest.waitForExplicitFinish();
@@ -39,7 +39,7 @@ SimpleTest.waitForFocus(function() {
       SimpleTest.executeSoon(function() {
         synthesizeKey("KEY_Backspace");
 
-        onSpellCheck(div, function() {
+        maybeOnSpellCheck(div, function() {
           var sel = getSpellCheckSelection();
           is(sel.rangeCount, 2, "We should have two misspelled words");
           is(String(sel.getRangeAt(0)), "fivee", "Correct misspelled word");
diff --git a/editor/spellchecker/tests/test_bug1154791.html b/editor/spellchecker/tests/test_bug1154791.html
index 8ac00104b854..7fa91a7023b8 100644
--- a/editor/spellchecker/tests/test_bug1154791.html
+++ b/editor/spellchecker/tests/test_bug1154791.html
@@ -22,7 +22,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1154791
 
 <script class="testbody" type="application/javascript">
 
-let {onSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
+let {maybeOnSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
 
 /** Test for Bug 1154791 **/
 SimpleTest.waitForExplicitFinish();
@@ -39,7 +39,7 @@ SimpleTest.waitForFocus(function() {
       SimpleTest.executeSoon(function() {
         sendString(" ");
 
-        onSpellCheck(div, function() {
+        maybeOnSpellCheck(div, function() {
           var sel = getSpellCheckSelection();
           is(sel.rangeCount, 2, "We should have two misspelled words");
           is(String(sel.getRangeAt(0)), "thiss", "Correct misspelled word");
diff --git a/editor/spellchecker/tests/test_bug1200533.html b/editor/spellchecker/tests/test_bug1200533.html
index 107e54cb4e2a..b461388a2425 100644
--- a/editor/spellchecker/tests/test_bug1200533.html
+++ b/editor/spellchecker/tests/test_bug1200533.html
@@ -108,13 +108,14 @@ function continueTest(evt) {
 
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm")
-  .onSpellCheck(elem, function() {
+  .onSpellCheck(elem, async function() {
     var spellchecker = inlineSpellChecker.spellChecker;
+    let currentDictionaries;
     try {
-      var dict = spellchecker.GetCurrentDictionary();
+      currentDictionaries = spellchecker.getCurrentDictionaries();
     } catch (e) {}
 
-    if (!dict && !retrying) {
+    if (!currentDictionaries && !retrying) {
       // It's possible for an asynchronous font-list update to cause a reflow
       // that disrupts the async spell-check and results in not getting a
       // current dictionary here; if that happens, we retry the same testcase
@@ -122,6 +123,8 @@ function continueTest(evt) {
       info(`No current dictionary: retrying testcase ${loadCount}`);
       retrying = true;
     } else {
+      is(currentDictionaries.length, 1, "expected one dictionary");
+      let dict = currentDictionaries[0];
       if (tests[loadCount][2] != "*") {
         is(dict, tests[loadCount][2], "expected " + tests[loadCount][2]);
       } else if (is_en_US && tests[loadCount][0].startsWith("en")) {
@@ -142,7 +145,7 @@ function continueTest(evt) {
       content.src = "http://mochi.test:8888/tests/editor/spellchecker/tests/bug1200533_subframe.html?firstload=false";
     } else {
       // Remove the fake  dictionaries again, since it's otherwise picked up by later tests.
-      script.sendAsyncMessage("destroy");
+      await script.sendQuery("destroy");
 
       SimpleTest.finish();
     }
diff --git a/editor/spellchecker/tests/test_bug1204147.html b/editor/spellchecker/tests/test_bug1204147.html
index e009e1c43264..ba59e3b0ee71 100644
--- a/editor/spellchecker/tests/test_bug1204147.html
+++ b/editor/spellchecker/tests/test_bug1204147.html
@@ -71,17 +71,16 @@ var loadListener = async function(evt) {
 
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm")
-  .onSpellCheck(elem, function() {
-    var spellchecker = inlineSpellChecker.spellChecker;
-    try {
-      var currentDictonary = spellchecker.GetCurrentDictionary();
-    } catch (e) {}
-
+  .onSpellCheck(elem, async function() {
+    let spellchecker = inlineSpellChecker.spellChecker;
+    let currentDictionaries = spellchecker.getCurrentDictionaries();
+    is(currentDictionaries.length, 1, "expected one dictionary");
+    let currentDictionary = currentDictionaries[0];
     if (firstLoad) {
       firstLoad = false;
 
        // First time around, the element's language should be used.
-      is(currentDictonary, "en-GB", "unexpected lang " + currentDictonary + " instead of en-GB");
+      is(currentDictionary, "en-GB", "unexpected lang " + currentDictionary + " instead of en-GB");
 
       // Note that on second load, we load a different page, which does NOT have the trouble-causing
       // contenteditable in it. Sadly, loading the same page with the trouble-maker in it
@@ -93,11 +92,11 @@ var loadListener = async function(evt) {
     } else {
       // Second time around, the element should default to en-US.
       // Without the fix, the first run sets the content preference to en-GB for the whole site.
-      is(currentDictonary, "en-US", "unexpected lang " + currentDictonary + " instead of en-US");
+      is(currentDictionary, "en-US", "unexpected lang " + currentDictionary + " instead of en-US");
       content.removeEventListener("load", loadListener);
 
       // Remove the fake en-GB dictionary again, since it's otherwise picked up by later tests.
-      script.sendAsyncMessage("destroy");
+      await script.sendQuery("destroy");
 
       // Reset the preference, so the last value we set doesn't collide with the next test.
       SimpleTest.finish();
diff --git a/editor/spellchecker/tests/test_bug1205983.html b/editor/spellchecker/tests/test_bug1205983.html
index e85bb5efdb67..a0ae3220afc8 100644
--- a/editor/spellchecker/tests/test_bug1205983.html
+++ b/editor/spellchecker/tests/test_bug1205983.html
@@ -28,9 +28,9 @@ var editor_de;
 var selcon_de;
 var script;
 
-var onSpellCheck =
+var maybeOnSpellCheck =
   SpecialPowers.Cu.import(
-    "resource://testing-common/AsyncSpellCheckTestHelper.jsm").onSpellCheck;
+    "resource://testing-common/AsyncSpellCheckTestHelper.jsm").maybeOnSpellCheck;
 
 /** Test for Bug 1205983 **/
 SimpleTest.waitForExplicitFinish();
@@ -66,7 +66,7 @@ SimpleTest.waitForFocus(async function() {
 function deFocus() {
   elem_de = document.getElementById("de-DE");
 
-  onSpellCheck(elem_de, function() {
+  maybeOnSpellCheck(elem_de, function() {
     var editingSession = SpecialPowers.wrap(window).docShell.editingSession;
     editor_de = editingSession.getEditorForWindow(window);
     selcon_de = editor_de.selectionController;
@@ -88,15 +88,14 @@ function enFocus() {
   editor_en.setSpellcheckUserOverride(true);
   var inlineSpellChecker = editor_en.getInlineSpellChecker(true);
 
-  onSpellCheck(elem_en, function() {
+  maybeOnSpellCheck(elem_en, async function() {
     var spellchecker = inlineSpellChecker.spellChecker;
-    let currentDictonary;
-    try {
-      currentDictonary = spellchecker.GetCurrentDictionary();
-    } catch (e) {}
+    let currentDictionaries = spellchecker.getCurrentDictionaries();
+    is(currentDictionaries.length, 1, "expected one dictionary");
+    let currentDictionary = currentDictionaries[0];
 
     // Check that the English dictionary is loaded and that the spell check has worked.
-    is(currentDictonary, "en-US", "expected en-US");
+    is(currentDictionary, "en-US", "expected en-US");
     is(getMisspelledWords(editor_en), "Nogoodword", "one misspelled word expected: Nogoodword");
 
     // So far all was boring. The important thing is whether the spell check result
@@ -106,7 +105,7 @@ function enFocus() {
     is(sel.toString(), "German", "one misspelled word expected: German");
 
     // Remove the fake de_DE dictionary again.
-    script.sendAsyncMessage("destroy");
+    await script.sendQuery("destroy");
 
     // Focus again, so the spelling gets updated, but before we need to kill the focus handler.
     elem_de.onfocus = null;
@@ -114,7 +113,7 @@ function enFocus() {
     elem_de.focus();
 
     // After removal, the de_DE editor should refresh the spelling with en-US.
-    onSpellCheck(elem_de, function() {
+    maybeOnSpellCheck(elem_de, function() {
       var endSel = selcon_de.getSelection(selcon_de.SELECTION_SPELLCHECK);
       // eslint-disable-next-line no-useless-concat
       is(endSel.toString(), "heute" + "ist" + "ein" + "guter",
diff --git a/editor/spellchecker/tests/test_bug1209414.html b/editor/spellchecker/tests/test_bug1209414.html
index aeb9b822744c..ad0c039da579 100644
--- a/editor/spellchecker/tests/test_bug1209414.html
+++ b/editor/spellchecker/tests/test_bug1209414.html
@@ -32,13 +32,12 @@ var script;
 /** Test for Bug 1209414 **/
 /*
  * All we want to do in this test is change the spelling using a right-click and selection from the menu.
- * This is necessary since all the other tests use SetCurrentDictionary() which doesn't reflect
+ * This is necessary since all the other tests use setCurrentDictionaries() which doesn't reflect
  * user behaviour.
  */
 
-var onSpellCheck =
-  SpecialPowers.Cu.import(
-    "resource://testing-common/AsyncSpellCheckTestHelper.jsm").onSpellCheck;
+let {maybeOnSpellCheck, onSpellCheck} = SpecialPowers.Cu.import(
+    "resource://testing-common/AsyncSpellCheckTestHelper.jsm")
 
 SimpleTest.waitForExplicitFinish();
 SimpleTest.waitForFocus(async function() {
@@ -73,11 +72,9 @@ SimpleTest.waitForFocus(async function() {
 
       // Select Language from the menu.  Take a look at
       // toolkit/modules/InlineSpellChecker.jsm to see how the menu works.
-
       contextMenu.ownerDocument.getElementById("spell-check-dictionary-en-US")
                  .doCommand();
       contextMenu.hidePopup();
-
       return state;
     });
     addMessageListener("destroy", () => hunspell.removeDirectory(de_DE));
@@ -94,15 +91,14 @@ SimpleTest.waitForFocus(async function() {
   editor_de = SpecialPowers.wrap(elem_de).editor;
   editor_de.setSpellcheckUserOverride(true);
 
-  onSpellCheck(elem_de, function() {
+  maybeOnSpellCheck(elem_de, function() {
     var inlineSpellChecker = editor_de.getInlineSpellChecker(true);
     var spellchecker = inlineSpellChecker.spellChecker;
-    try {
-      var currentDictonary = spellchecker.GetCurrentDictionary();
-    } catch (e) {}
+    let currentDictionaries = spellchecker.getCurrentDictionaries();
 
     // Check that the German dictionary is loaded and that the spell check has worked.
-    is(currentDictonary, "de-DE", "expected de-DE");
+    is(currentDictionaries.length, 1, "expected one dictionary");
+    is(currentDictionaries[0], "de-DE", "expected de-DE");
     // eslint-disable-next-line no-useless-concat
     is(getMisspelledWords(editor_de), "today" + "is" + "a" + "good" + "day", "some misspelled words expected: today is a good day");
 
@@ -110,7 +106,7 @@ SimpleTest.waitForFocus(async function() {
     elem_de.focus();
 
     // Make sure all spell checking action is done before right-click to select the en-US dictionary.
-    onSpellCheck(elem_de, function() {
+    maybeOnSpellCheck(elem_de, function() {
       synthesizeMouse(elem_de, 2, 2, { type: "contextmenu", button: 2 }, window);
     });
   });
@@ -120,25 +116,24 @@ async function handlePopup() {
   var state = await script.sendQuery("hidepopup");
   is(state, "open", "checking if popup is open");
 
-  onSpellCheck(elem_de, function() {
+  onSpellCheck(elem_de, async function() {
     var inlineSpellChecker = editor_de.getInlineSpellChecker(true);
     var spellchecker = inlineSpellChecker.spellChecker;
-    let currentDictonary;
-    try {
-      currentDictonary = spellchecker.GetCurrentDictionary();
-    } catch (e) {}
+    let currentDictionaries = spellchecker.getCurrentDictionaries();
 
     // Check that the English dictionary is loaded and that the spell check has worked.
-    is(currentDictonary, "en-US", "expected en-US");
-    // eslint-disable-next-line no-useless-concat
-    is(getMisspelledWords(editor_de), "heute" + "ist" + "ein" + "guter", "some misspelled words expected: heute ist ein guter");
+    is(currentDictionaries.length, 2, "expected two dictionaries");
+    is(currentDictionaries[0], "de-DE", "expected de-DE");
+    is(currentDictionaries[1], "en-US", "expected en-US");
+    is(getMisspelledWords(editor_de), "", "No misspelled words expected");
 
     // Remove the fake de_DE dictionary again.
-    script.sendAsyncMessage("destroy");
+    await script.sendQuery("destroy");
 
     // This will clear the content preferences and reset "spellchecker.dictionary".
-    spellchecker.SetCurrentDictionary("");
-    SimpleTest.finish();
+    spellchecker.setCurrentDictionaries([]).then(() => {
+      SimpleTest.finish();
+    });
   });
 }
 
diff --git a/editor/spellchecker/tests/test_bug1219928.html b/editor/spellchecker/tests/test_bug1219928.html
index bbd8f80cd797..2fc960aabba7 100644
--- a/editor/spellchecker/tests/test_bug1219928.html
+++ b/editor/spellchecker/tests/test_bug1219928.html
@@ -29,14 +29,14 @@ var spellchecker;
 
 SimpleTest.waitForExplicitFinish();
 SimpleTest.waitForFocus(function() {
-  var onSpellCheck =
+  var maybeOnSpellCheck =
     SpecialPowers.Cu.import(
-      "resource://testing-common/AsyncSpellCheckTestHelper.jsm", null).onSpellCheck;
+      "resource://testing-common/AsyncSpellCheckTestHelper.jsm", {}).maybeOnSpellCheck;
 
   var elem = document.getElementById("en-US");
   elem.focus();
 
-  onSpellCheck(elem, function() {
+  maybeOnSpellCheck(elem, function() {
     var editingSession = SpecialPowers.wrap(window).docShell.editingSession;
     var editor = editingSession.getEditorForWindow(window);
     var selcon = editor.selectionController;
diff --git a/editor/spellchecker/tests/test_bug1418629.html b/editor/spellchecker/tests/test_bug1418629.html
index 168983f97cf6..fa571536fe31 100644
--- a/editor/spellchecker/tests/test_bug1418629.html
+++ b/editor/spellchecker/tests/test_bug1418629.html
@@ -19,7 +19,7 @@
 <div id="edit1" contenteditable=true></div>
 
 <script>
-const {onSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
+const {maybeOnSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
 
 SimpleTest.waitForExplicitFinish();
 
@@ -52,7 +52,7 @@ async function test_with_single_quote(input) {
   synthesizeKey("e");
   synthesizeKey("s");
 
-  await new Promise((resolve) => { onSpellCheck(input, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(input, resolve); });
   let editor = getEditor(input);
   // isSpellingCheckOk is defined in spellcheck.js
   ok(isSpellingCheckOk(editor, misspeltWords), "no misspelt words");
@@ -61,7 +61,7 @@ async function test_with_single_quote(input) {
   synthesizeKey("\'");
   is(input.value || input.textContent, "doesn\'", "");
 
-  await new Promise((resolve) => { onSpellCheck(input, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(input, resolve); });
   // XXX This won't work since mozInlineSpellWordUtil::SplitDOM removes
   // last single quote unfortunately that is during inputting.
   // isSpellingCheckOk is defined in spellcheck.js
@@ -71,7 +71,7 @@ async function test_with_single_quote(input) {
   synthesizeKey(" ");
   is(input.value || input.textContent, "doesn\' ", "");
 
-  await new Promise((resolve) => { onSpellCheck(input, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(input, resolve); });
   misspeltWords.push("doesn");
   // isSpellingCheckOk is defined in spellcheck.js
   ok(isSpellingCheckOk(editor, misspeltWords), "should run spellchecker");
@@ -95,7 +95,7 @@ async function test_with_twice_characters(input, ch) {
   // trigger spellchecker
   synthesizeKey(" ");
 
-  await new Promise((resolve) => { onSpellCheck(input, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(input, resolve); });
   misspeltWords.push("doesn");
   let editor = getEditor(input);
   // isSpellingCheckOk is defined in spellcheck.js
@@ -115,7 +115,7 @@ async function test_between_single_quote(input) {
   synthesizeKey("t");
   synthesizeKey("\'");
 
-  await new Promise((resolve) => { onSpellCheck(input, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(input, resolve); });
   let editor = getEditor(input);
   ok(isSpellingCheckOk(editor, misspeltWords),
      "don't run spellchecker between single qoute");
@@ -141,14 +141,14 @@ async function test_with_email(input) {
   synthesizeKey("o");
   synthesizeKey("m");
 
-  await new Promise((resolve) => { onSpellCheck(input, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(input, resolve); });
   let editor = getEditor(input);
   ok(isSpellingCheckOk(editor, misspeltWords),
      "don't run spellchecker for email address");
 
   synthesizeKey(" ");
 
-  await new Promise((resolve) => { onSpellCheck(input, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(input, resolve); });
   ok(isSpellingCheckOk(editor, misspeltWords),
      "no misspelt words due to email address");
 }
@@ -175,14 +175,14 @@ async function test_with_url(input) {
   synthesizeKey("o");
   synthesizeKey("m");
 
-  await new Promise((resolve) => { onSpellCheck(input, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(input, resolve); });
   let editor = getEditor(input);
   ok(isSpellingCheckOk(editor, misspeltWords),
      "don't run spellchecker for URL");
 
   synthesizeKey(" ");
 
-  await new Promise((resolve) => { onSpellCheck(input, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(input, resolve); });
   ok(isSpellingCheckOk(editor, misspeltWords),
      "no misspelt words due to URL");
 }
diff --git a/editor/spellchecker/tests/test_bug1497480.html b/editor/spellchecker/tests/test_bug1497480.html
index cc18facf005d..7775c3c47ed0 100644
--- a/editor/spellchecker/tests/test_bug1497480.html
+++ b/editor/spellchecker/tests/test_bug1497480.html
@@ -23,8 +23,8 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1497480
 
 /** Test for Bug 1497480 **/
 let gMisspeltWords = [];
-let onSpellCheck = SpecialPowers.Cu.import(
-  "resource://testing-common/AsyncSpellCheckTestHelper.jsm", null).onSpellCheck;
+let maybeOnSpellCheck = SpecialPowers.Cu.import(
+  "resource://testing-common/AsyncSpellCheckTestHelper.jsm", null).maybeOnSpellCheck;
 
 const template = document.createElement("template");
 template.innerHTML = `<div id="target" contenteditable>Test</div>`;
@@ -52,7 +52,7 @@ add_task(async function() {
   // Wait for first full spell-checking.
   synthesizeMouseAtCenter(outOfTarget, {}, window);
   await new Promise((resolve) => {
-    onSpellCheck(outOfTarget, function() {
+    maybeOnSpellCheck(outOfTarget, function() {
       resolve();
     });
   });
@@ -65,7 +65,7 @@ add_task(async function() {
   gMisspeltWords.push("spellechek");
   synthesizeMouseAtCenter(outOfTarget, {}, window);
   await new Promise((resolve) => {
-    onSpellCheck(target, function() {
+    maybeOnSpellCheck(target, function() {
       ok(isSpellingCheckOk(getEditor(), gMisspeltWords),
          "Spell-checking should be performed when anchor navigates away from ShadowDOM");
       SimpleTest.executeSoon(resolve);
@@ -80,7 +80,7 @@ add_task(async function() {
   gMisspeltWords.push("spellechck");
   synthesizeKey("KEY_Enter", {}, window);
   await new Promise((resolve) => {
-    onSpellCheck(target, function() {
+    maybeOnSpellCheck(target, function() {
       ok(isSpellingCheckOk(getEditor(), gMisspeltWords),
          "Spell-checking should be performed when pressing enter in contenteditable in ShadowDOM");
       SimpleTest.executeSoon(resolve);
diff --git a/editor/spellchecker/tests/test_bug1602526.html b/editor/spellchecker/tests/test_bug1602526.html
index 9a2097d251e5..6d69762a6e0c 100644
--- a/editor/spellchecker/tests/test_bug1602526.html
+++ b/editor/spellchecker/tests/test_bug1602526.html
@@ -17,7 +17,7 @@
 <div id="contenteditable" contenteditable=true>kkkk&#xf6;kkkk</div>
 
 <script>
-const {onSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
+const {maybeOnSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
 
 SimpleTest.waitForExplicitFinish();
 
@@ -36,13 +36,13 @@ SimpleTest.waitForFocus(async () => {
   synthesizeKey(" ");
 
   // Run spell checker
-  await new Promise((resolve) => { onSpellCheck(contenteditable, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(contenteditable, resolve); });
 
   synthesizeKey("a");
   synthesizeKey("a");
   synthesizeKey("a");
 
-  await new Promise((resolve) => { onSpellCheck(contenteditable, resolve); });
+  await new Promise((resolve) => { maybeOnSpellCheck(contenteditable, resolve); });
   let editor = getEditor();
   // isSpellingCheckOk is defined in spellcheck.js
   // eslint-disable-next-line no-undef
diff --git a/editor/spellchecker/tests/test_bug338427.html b/editor/spellchecker/tests/test_bug338427.html
index a31645646099..2361e7043001 100644
--- a/editor/spellchecker/tests/test_bug338427.html
+++ b/editor/spellchecker/tests/test_bug338427.html
@@ -35,18 +35,20 @@ function init() {
         ok(list.length > 0, "At least one dictionary should be present");
 
         var lang = list[0];
-        spellchecker.spellChecker.SetCurrentDictionary(lang);
-
-        onSpellCheck(textarea, function() {
+        spellchecker.spellChecker.setCurrentDictionaries([lang]).then(() => {
+          onSpellCheck(textarea, function() {
             try {
-                var dictionary =
-                    spellchecker.spellChecker.GetCurrentDictionary();
+                var dictionaries =
+                    spellchecker.spellChecker.getCurrentDictionaries();
             } catch (e) {}
-            is(dictionary, lang, "Unexpected spell check dictionary");
+            is(dictionaries.length, 1, "Expected one dictionary");
+            is(dictionaries[0], lang, "Unexpected spell check dictionary");
 
             // This will clear the content preferences and reset "spellchecker.dictionary".
-            spellchecker.spellChecker.SetCurrentDictionary("");
-            SimpleTest.finish();
+            spellchecker.spellChecker.setCurrentDictionaries([]).then(() => {
+              SimpleTest.finish();
+            });
+          });
         });
     });
 }
diff --git a/editor/spellchecker/tests/test_bug366682.html b/editor/spellchecker/tests/test_bug366682.html
index a6f5135a4b18..72cbe1119d29 100644
--- a/editor/spellchecker/tests/test_bug366682.html
+++ b/editor/spellchecker/tests/test_bug366682.html
@@ -47,7 +47,7 @@ function runTest() {
 
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm")
-               .onSpellCheck(editDoc().documentElement, evalTest);
+               .maybeOnSpellCheck(editDoc().documentElement, evalTest);
 }
 
 function evalTest() {
diff --git a/editor/spellchecker/tests/test_bug432225.html b/editor/spellchecker/tests/test_bug432225.html
index 010c1cdf4bc6..9f74ec60197a 100644
--- a/editor/spellchecker/tests/test_bug432225.html
+++ b/editor/spellchecker/tests/test_bug432225.html
@@ -21,7 +21,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=432225
 
 /** Test for Bug 432225 **/
 
-let {onSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
+let {maybeOnSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
 
 SimpleTest.waitForExplicitFinish();
 addLoadEvent(runTest);
@@ -57,7 +57,7 @@ function addWords(aLimit) {
   getEdit().focus();
   sendString("aa OK ");
   gMisspeltWords.push("aa");
-  onSpellCheck(editDoc(), function() {
+  maybeOnSpellCheck(editDoc(), function() {
     addWords(aLimit - 1);
   });
 }
diff --git a/editor/spellchecker/tests/test_bug484181.html b/editor/spellchecker/tests/test_bug484181.html
index 50dfe2d50dab..7436c812ded9 100644
--- a/editor/spellchecker/tests/test_bug484181.html
+++ b/editor/spellchecker/tests/test_bug484181.html
@@ -46,14 +46,14 @@ function runTest() {
   var edit = document.getElementById("edit");
   edit.focus();
 
-  const {onSpellCheck} = SpecialPowers.Cu.import(
+  const {maybeOnSpellCheck} = SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm", null);
-  onSpellCheck(edit, function() {
+  maybeOnSpellCheck(edit, function() {
     ok(isSpellingCheckOk(getEditor(), gMisspeltWords),
        "All misspellings before editing are accounted for.");
 
     append(" becaz I'm a lulcat!");
-    onSpellCheck(edit, function() {
+    maybeOnSpellCheck(edit, function() {
       gMisspeltWords.push("becaz");
       gMisspeltWords.push("lulcat");
       ok(isSpellingCheckOk(getEditor(), gMisspeltWords),
diff --git a/editor/spellchecker/tests/test_bug678842.html b/editor/spellchecker/tests/test_bug678842.html
index a2b9c8db2303..912c33cf002a 100644
--- a/editor/spellchecker/tests/test_bug678842.html
+++ b/editor/spellchecker/tests/test_bug678842.html
@@ -64,36 +64,33 @@ var loadListener = async function(evt) {
 
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm")
-  .onSpellCheck(elem, function() {
-    var spellchecker = inlineSpellChecker.spellChecker;
-    try {
-      var currentDictonary = spellchecker.GetCurrentDictionary();
-    } catch (e) {}
-
-    if (!currentDictonary) {
-      spellchecker.SetCurrentDictionary("en-US");
-    }
+  .onSpellCheck(elem, async function() {
+    let spellchecker = inlineSpellChecker.spellChecker;
+    let currentDictionaries = spellchecker.getCurrentDictionaries();
+    is(currentDictionaries.length, 1, "expected one dictionary");
+    let currentDictionary = currentDictionaries[0];
 
     if (firstLoad) {
       firstLoad = false;
 
        // First time around, the dictionary defaults to the locale.
-      is(currentDictonary, "en-US", "unexpected lang " + currentDictonary + " instead of en-US");
+      is(currentDictionary, "en-US", "unexpected lang " + currentDictionary + " instead of en-US");
 
       // Select en-GB.
-      spellchecker.SetCurrentDictionary("en-GB");
-
-      content.src = "http://mochi.test:8888/tests/editor/spellchecker/tests/bug678842_subframe.html?firstload=false";
+      spellchecker.setCurrentDictionaries(["en-GB"]).then(() => {
+        content.src = "http://mochi.test:8888/tests/editor/spellchecker/tests/bug678842_subframe.html?firstload=false";
+      });
     } else {
-      is(currentDictonary, "en-GB", "unexpected lang " + currentDictonary + " instead of en-GB");
+      is(currentDictionary, "en-GB", "unexpected lang " + currentDictionary + " instead of en-GB");
       content.removeEventListener("load", loadListener);
 
       // Remove the fake en-GB dictionary again, since it's otherwise picked up by later tests.
-      script.sendAsyncMessage("destroy");
+      await script.sendQuery("destroy");
 
       // This will clear the content preferences and reset "spellchecker.dictionary".
-      spellchecker.SetCurrentDictionary("");
-      SimpleTest.finish();
+      spellchecker.setCurrentDictionaries([]).then( () => {
+        SimpleTest.finish();
+      });
     }
   });
 };
diff --git a/editor/spellchecker/tests/test_bug697981.html b/editor/spellchecker/tests/test_bug697981.html
index 7dcd1bca3a46..3a02b433adc9 100644
--- a/editor/spellchecker/tests/test_bug697981.html
+++ b/editor/spellchecker/tests/test_bug697981.html
@@ -27,6 +27,11 @@ var elem_de;
 var editor_de;
 var script;
 
+var maybeOnSpellCheck =
+  SpecialPowers.Cu.import(
+    "resource://testing-common/AsyncSpellCheckTestHelper.jsm")
+               .maybeOnSpellCheck;
+
 var onSpellCheck =
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm")
@@ -69,14 +74,15 @@ function deFocus() {
   editor_de.setSpellcheckUserOverride(true);
   var inlineSpellChecker = editor_de.getInlineSpellChecker(true);
 
-  onSpellCheck(elem_de, function() {
+  maybeOnSpellCheck(elem_de, function() {
     var spellchecker = inlineSpellChecker.spellChecker;
     try {
-      var currentDictonary = spellchecker.GetCurrentDictionary();
+      var currentDictionaries = spellchecker.getCurrentDictionaries();
     } catch (e) {}
 
     // Check that the German dictionary is loaded and that the spell check has worked.
-    is(currentDictonary, "de-DE", "expected de-DE");
+    is(currentDictionaries.length, 1, "expected one dictionary");
+    is(currentDictionaries[0], "de-DE", "expected de-DE");
     is(getMisspelledWords(editor_de), "German", "one misspelled word expected: German");
 
     // Now focus the other textarea, which requires English spelling.
@@ -90,15 +96,13 @@ function enFocus() {
   editor_en.setSpellcheckUserOverride(true);
   var inlineSpellChecker = editor_en.getInlineSpellChecker(true);
 
-  onSpellCheck(elem_en, function() {
-    var spellchecker = inlineSpellChecker.spellChecker;
-    let currentDictonary;
-    try {
-      currentDictonary = spellchecker.GetCurrentDictionary();
-    } catch (e) {}
+  onSpellCheck(elem_en, async function() {
+    let spellchecker = inlineSpellChecker.spellChecker;
+    let currentDictionaries = spellchecker.getCurrentDictionaries();
 
     // Check that the English dictionary is loaded and that the spell check has worked.
-    is(currentDictonary, "en-US", "expected en-US");
+    is(currentDictionaries.length, 1, "expected one dictionary");
+    is(currentDictionaries[0], "en-US", "expected en-US");
     is(getMisspelledWords(editor_en), "Nogoodword", "one misspelled word expected: Nogoodword");
 
     // So far all was boring. The important thing is whether the spell check result
@@ -107,7 +111,7 @@ function enFocus() {
     is(getMisspelledWords(editor_de), "German", "one misspelled word expected: German");
 
     // Remove the fake de_DE dictionary again.
-    script.sendAsyncMessage("destroy");
+    await script.sendQuery("destroy");
 
     // Focus again, so the spelling gets updated, but before we need to kill the focus handler.
     elem_de.onfocus = null;
@@ -115,14 +119,15 @@ function enFocus() {
     elem_de.focus();
 
     // After removal, the de_DE editor should refresh the spelling with en-US.
-    onSpellCheck(elem_de, function() {
+    maybeOnSpellCheck(elem_de, function() {
       spellchecker = inlineSpellChecker.spellChecker;
       try {
-        currentDictonary = spellchecker.GetCurrentDictionary();
+        currentDictionaries = spellchecker.getCurrentDictionaries();
       } catch (e) {}
 
       // Check that the default English dictionary is loaded and that the spell check has worked.
-      is(currentDictonary, "en-US", "expected en-US");
+      is(currentDictionaries.length, 1, "expected one dictionary");
+      is(currentDictionaries[0], "en-US", "expected en-US");
       // eslint-disable-next-line no-useless-concat
       is(getMisspelledWords(editor_de), "heute" + "ist" + "ein" + "guter",
          "some misspelled words expected: heute ist ein guter");
diff --git a/editor/spellchecker/tests/test_bug717433.html b/editor/spellchecker/tests/test_bug717433.html
index 3c55ace1280d..8dd082574418 100644
--- a/editor/spellchecker/tests/test_bug717433.html
+++ b/editor/spellchecker/tests/test_bug717433.html
@@ -65,36 +65,37 @@ var loadListener = async function(evt) {
 
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm")
-  .onSpellCheck(elem, function() {
-    var spellchecker = inlineSpellChecker.spellChecker;
-    try {
-      var currentDictonary = spellchecker.GetCurrentDictionary();
-    } catch (e) {}
+  .onSpellCheck(elem, async function() {
+    let spellchecker = inlineSpellChecker.spellChecker;
+    let currentDictionaries = spellchecker.getCurrentDictionaries();
+
+    is(currentDictionaries.length, 1, "expected one dictionary");
+    let currentDictionary = currentDictionaries[0];
 
     if (firstLoad) {
       firstLoad = false;
 
       // First time around, we get a random dictionary based on the language "en".
-      if (currentDictonary == "en-GB") {
-        spellchecker.SetCurrentDictionary("en-US");
+      if (currentDictionary == "en-GB") {
         expected = "en-US";
-      } else if (currentDictonary == "en-US") {
-        spellchecker.SetCurrentDictionary("en-GB");
+      } else if (currentDictionary == "en-US") {
         expected = "en-GB";
       } else {
         is(true, false, "Neither en-US nor en-GB are current");
       }
-      content.src = "http://mochi.test:8888/tests/editor/spellchecker/tests/bug717433_subframe.html?firstload=false";
+      spellchecker.setCurrentDictionaries([expected]).then(() => {
+        content.src = "http://mochi.test:8888/tests/editor/spellchecker/tests/bug717433_subframe.html?firstload=false";});
     } else {
-      is(currentDictonary, expected, expected + " expected");
+      is(currentDictionary, expected, expected + " expected");
       content.removeEventListener("load", loadListener);
 
       // Remove the fake en-GB dictionary again, since it's otherwise picked up by later tests.
-      script.sendAsyncMessage("destroy");
+      await script.sendQuery("destroy");
 
       // This will clear the content preferences and reset "spellchecker.dictionary".
-      spellchecker.SetCurrentDictionary("");
-      SimpleTest.finish();
+      spellchecker.setCurrentDictionaries([]).then(() => {
+        SimpleTest.finish();
+      });
     }
   });
 };
diff --git a/editor/spellchecker/tests/test_nsIEditorSpellCheck_ReplaceWord.html b/editor/spellchecker/tests/test_nsIEditorSpellCheck_ReplaceWord.html
index 440dbda01de1..428870d90063 100644
--- a/editor/spellchecker/tests/test_nsIEditorSpellCheck_ReplaceWord.html
+++ b/editor/spellchecker/tests/test_nsIEditorSpellCheck_ReplaceWord.html
@@ -12,12 +12,12 @@
 
 SimpleTest.waitForExplicitFinish();
 SimpleTest.waitForFocus(async () => {
-  const { onSpellCheck } = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm");
+  const { maybeOnSpellCheck } = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm");
   const editor = document.querySelector("div[contenteditable]");
   async function replaceWord(aMisspelledWord, aCorrectWord, aReplaceAll) {
     const editorObj = SpecialPowers.wrap(window).docShell.editingSession.getEditorForWindow(window);
     const inlineSpellChecker = editorObj.getInlineSpellChecker(true);
-    await new Promise(resolve => onSpellCheck(editor, resolve));
+    await new Promise(resolve => maybeOnSpellCheck(editor, resolve));
     const editorSpellCheck = inlineSpellChecker.spellChecker;
     editorObj.beginTransaction();
     try {
diff --git a/editor/spellchecker/tests/test_spellcheck_after_edit.html b/editor/spellchecker/tests/test_spellcheck_after_edit.html
index c578c415f2a3..181cd1163dcf 100644
--- a/editor/spellchecker/tests/test_spellcheck_after_edit.html
+++ b/editor/spellchecker/tests/test_spellcheck_after_edit.html
@@ -8,7 +8,7 @@
 </head>
 <body>
 <script>
-let {onSpellCheck} =
+let {maybeOnSpellCheck} =
   SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
 
 function waitForTick() {
@@ -30,7 +30,7 @@ async function waitForOnSpellCheck(
   info(`Waiting for onSpellCheck (${aWhen})...`);
   for (let retry = 0; retry < 100; retry++) {
     await waitForTick();
-    await new Promise(resolve => onSpellCheck(aEditingHost, resolve));
+    await new Promise(resolve => maybeOnSpellCheck(aEditingHost, resolve));
     if (aWaitForNumberOfMisspelledWords === 0) {
       if (aSpellCheckSelection.rangeCount === 0) {
         break;
diff --git a/editor/spellchecker/tests/test_spellcheck_after_pressing_navigation_key.html b/editor/spellchecker/tests/test_spellcheck_after_pressing_navigation_key.html
index 2e0c9bd4db4e..5a8ff059e6b9 100644
--- a/editor/spellchecker/tests/test_spellcheck_after_pressing_navigation_key.html
+++ b/editor/spellchecker/tests/test_spellcheck_after_pressing_navigation_key.html
@@ -13,7 +13,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1729653
 <body>
 <textarea rows="20" cols="50">That undfgdfg seems OK.</textarea>
 <script>
-let {onSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
+let {maybeOnSpellCheck} = SpecialPowers.Cu.import("resource://testing-common/AsyncSpellCheckTestHelper.jsm", {});
 
 function waitForTick() {
   return new Promise(resolve => SimpleTest.executeSoon(resolve));
@@ -21,7 +21,7 @@ function waitForTick() {
 
 function waitForOnSpellCheck(aTextArea) {
   info("Waiting for onSpellCheck...");
-  return new Promise(resolve => onSpellCheck(aTextArea, resolve));
+  return new Promise(resolve => maybeOnSpellCheck(aTextArea, resolve));
 }
 
 /** Test for Bug 1729653 **/
diff --git a/editor/spellchecker/tests/test_suggest.html b/editor/spellchecker/tests/test_suggest.html
index a29e028ece1a..4bd71aed2046 100644
--- a/editor/spellchecker/tests/test_suggest.html
+++ b/editor/spellchecker/tests/test_suggest.html
@@ -15,14 +15,14 @@
 add_task(async function() {
   await new Promise(resolve => SimpleTest.waitForFocus(resolve));
 
-  let { onSpellCheck } =
+  let { maybeOnSpellCheck } =
     SpecialPowers.Cu.import(
       "resource://testing-common/AsyncSpellCheckTestHelper.jsm", null);
 
   let element = document.getElementById("en-US");
   element.focus();
 
-  await new Promise(resolve => onSpellCheck(element, resolve));
+  await new Promise(resolve => maybeOnSpellCheck(element, resolve));
 
   let editingSession = SpecialPowers.wrap(window).docShell.editingSession;
   let editor = editingSession.getEditorForWindow(window);
diff --git a/extensions/spellcheck/hunspell/tests/unit/test_hunspell.js b/extensions/spellcheck/hunspell/tests/unit/test_hunspell.js
index 77ff1635e845..28736fc0235f 100644
--- a/extensions/spellcheck/hunspell/tests/unit/test_hunspell.js
+++ b/extensions/spellcheck/hunspell/tests/unit/test_hunspell.js
@@ -188,7 +188,7 @@ function do_run_test(checker, name, charset, todo_good, todo_bad) {
   Assert.ok(good.exists() || bad.exists() || sug.exists());
 
   dump("Setting dictionary to " + name + "\n");
-  checker.dictionary = name;
+  checker.dictionaries = [name];
 
   if (good.exists()) {
     var good_counter = 0;
diff --git a/extensions/spellcheck/hunspell/tests/unit/test_hunspell_unicode_paths.js b/extensions/spellcheck/hunspell/tests/unit/test_hunspell_unicode_paths.js
index 019f854291a1..655c42e70da3 100644
--- a/extensions/spellcheck/hunspell/tests/unit/test_hunspell_unicode_paths.js
+++ b/extensions/spellcheck/hunspell/tests/unit/test_hunspell_unicode_paths.js
@@ -34,7 +34,7 @@ add_task(async function() {
   await OS.File.writeAtomic(affPath, new TextEncoder().encode(""));
 
   spellCheck.loadDictionariesFromDir(baseDir);
-  spellCheck.dictionary = "dict";
+  spellCheck.dictionaries = ["dict"];
 
   ok(
     spellCheck.check(WORD),
diff --git a/extensions/spellcheck/tests/chrome/test_add_remove_dictionaries.xhtml b/extensions/spellcheck/tests/chrome/test_add_remove_dictionaries.xhtml
index 0ade475bafa2..ae3f42aa679d 100644
--- a/extensions/spellcheck/tests/chrome/test_add_remove_dictionaries.xhtml
+++ b/extensions/spellcheck/tests/chrome/test_add_remove_dictionaries.xhtml
@@ -27,12 +27,12 @@ function getDictionaryList(editor) {
 
 function getCurrentDictionary(editor) {
   var spellchecker = editor.getInlineSpellChecker(true).spellChecker;
-  return spellchecker.GetCurrentDictionary();
+  return spellchecker.getCurrentDictionaries()[0];
 }
 
 function setCurrentDictionary(editor, dictionary) {
   var spellchecker = editor.getInlineSpellChecker(true).spellChecker;
-  spellchecker.SetCurrentDictionary(dictionary);
+  return spellchecker.setCurrentDictionaries([dictionary]);
 }
 
 function RunTest() {
@@ -65,9 +65,9 @@ function RunTest() {
   ok(map.exists());
   hunspell.addDirectory(map);
 
-  const {onSpellCheck} = ChromeUtils.import(
+  const {maybeOnSpellCheck} = ChromeUtils.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm");
-  onSpellCheck(textbox, function () {
+    maybeOnSpellCheck(textbox, function () {
 
     // test that base and map dictionaries are available
     var dicts = getDictionaryList(editor);
@@ -75,48 +75,48 @@ function RunTest() {
     isnot(dicts.indexOf("maputf"), -1, "map is available");
 
     // select base dictionary
-    setCurrentDictionary(editor, "base-utf");
+    setCurrentDictionary(editor, "base-utf").then(() => {
+      /* eslint-disable no-useless-concat */
+      maybeOnSpellCheck(textbox, function () {
+        // test that base dictionary is in use
+        is(getMisspelledWords(editor), "Frühstück" + "qwertyu", "base misspellings");
+        is(getCurrentDictionary(editor), "base-utf", "current dictionary");
 
-    /* eslint-disable no-useless-concat */
-    onSpellCheck(textbox, function () {
-      // test that base dictionary is in use
-      is(getMisspelledWords(editor), "Frühstück" + "qwertyu", "base misspellings");
-      is(getCurrentDictionary(editor), "base-utf", "current dictionary");
+        // select map dictionary
+        setCurrentDictionary(editor, "maputf").then(() => {
+          // Focus again, so the spelling gets updated.
+          textbox.blur();
+          textbox.focus();
 
-      // select map dictionary
-      setCurrentDictionary(editor, "maputf");
+          maybeOnSpellCheck(textbox, function () {
+            // test that map dictionary is in use
+            is(getMisspelledWords(editor), "created" + "imply" + "tomorrow" + "qwertyu", "map misspellings (1)");
+            is(getCurrentDictionary(editor), "maputf", "current dictionary");
 
-      // Focus again, so the spelling gets updated.
-      textbox.blur();
-      textbox.focus();
+            // uninstall map dictionary
+            hunspell.removeDirectory(map);
 
-      onSpellCheck(textbox, function () {
-        // test that map dictionary is in use
-        is(getMisspelledWords(editor), "created" + "imply" + "tomorrow" + "qwertyu", "map misspellings (1)");
-        is(getCurrentDictionary(editor), "maputf", "current dictionary");
+            // Focus again, so the spelling gets updated.
+            textbox.blur();
+            textbox.focus();
 
-        // uninstall map dictionary
-        hunspell.removeDirectory(map);
+            maybeOnSpellCheck(textbox, function () {
+              // test that map dictionary is not in use
+              isnot(getMisspelledWords(editor), "created" + "imply" + "tomorrow" + "qwertyu", "map misspellings (2)");
+              isnot(getCurrentDictionary(editor), "maputf", "current dictionary");
 
-        // Focus again, so the spelling gets updated.
-        textbox.blur();
-        textbox.focus();
+              // test that base dictionary is available and map dictionary is unavailable
+              var dicts = getDictionaryList(editor);
+              isnot(dicts.indexOf("base-utf"), -1, "base is available");
+              is(dicts.indexOf("maputf"), -1, "map is unavailable");
 
-        onSpellCheck(textbox, function () {
-          // test that map dictionary is not in use
-          isnot(getMisspelledWords(editor), "created" + "imply" + "tomorrow" + "qwertyu", "map misspellings (2)");
-          isnot(getCurrentDictionary(editor), "maputf", "current dictionary");
+              // uninstall base dictionary
+              hunspell.removeDirectory(base);
 
-          // test that base dictionary is available and map dictionary is unavailable
-          var dicts = getDictionaryList(editor);
-          isnot(dicts.indexOf("base-utf"), -1, "base is available");
-          is(dicts.indexOf("maputf"), -1, "map is unavailable");
-
-          // uninstall base dictionary
-          hunspell.removeDirectory(base);
-
-          onSpellCheck(textbox, function () {
-            SimpleTest.finish();
+              maybeOnSpellCheck(textbox, function () {
+                SimpleTest.finish();
+              });
+            });
           });
         });
       });
diff --git a/extensions/spellcheck/tests/mochitest/test_bug1170484.html b/extensions/spellcheck/tests/mochitest/test_bug1170484.html
index e32fcf4c3cb2..77dd5aa75b85 100644
--- a/extensions/spellcheck/tests/mochitest/test_bug1170484.html
+++ b/extensions/spellcheck/tests/mochitest/test_bug1170484.html
@@ -14,7 +14,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1170484
   /** Test for Bug 1170484 **/
   SimpleTest.waitForExplicitFinish();
 
-  /* global onSpellCheck */
+  /* global maybeOnSpellCheck */
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm", window);
 
@@ -30,7 +30,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1170484
     // Not sure why, but postponing through the event loop seems to be enough.
     SimpleTest.executeSoon(function() {
       synthesizeMouseAtCenter($('misspelled'), {}, window);
-      onSpellCheck($('area'), function() {
+      maybeOnSpellCheck($('area'), function() {
         synthesizeMouseAtCenter($('misspelled'), {type: 'contextmenu'}, window);
         var mm = SpecialPowers.loadChromeScript(SimpleTest.getTestFileURL('helper_bug1170484.js'))
         info("Loaded chrome script");
diff --git a/extensions/spellcheck/tests/mochitest/test_bug1272623.html b/extensions/spellcheck/tests/mochitest/test_bug1272623.html
index 2d0830fa5ca3..de43c7e12c6e 100644
--- a/extensions/spellcheck/tests/mochitest/test_bug1272623.html
+++ b/extensions/spellcheck/tests/mochitest/test_bug1272623.html
@@ -29,7 +29,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1272623
 
   async function performCorrection(misspelled, area) {
     synthesizeMouseAtCenter($(misspelled), {}, window);
-    await new Promise(resolve => onSpellCheck($(area), resolve));
+    await new Promise(resolve => maybeOnSpellCheck($(area), resolve));
     synthesizeMouseAtCenter($(misspelled), {type: 'contextmenu'}, window);
 
     // Perform the spelling correction
@@ -61,7 +61,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1272623
   }
 
   add_task(async function() {
-    /* global onSpellCheck */
+    /* global maybeOnSpellCheck */
     SpecialPowers.Cu.import(
       "resource://testing-common/AsyncSpellCheckTestHelper.jsm", window);
 
diff --git a/layout/base/tests/bug512295-1-ref.html b/layout/base/tests/bug512295-1-ref.html
index 4947aab8cb58..64fd2df77450 100644
--- a/layout/base/tests/bug512295-1-ref.html
+++ b/layout/base/tests/bug512295-1-ref.html
@@ -14,7 +14,7 @@ x
   div.focus();
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm", window);
-  onSpellCheck(div, function () {
+  maybeOnSpellCheck(div, function () {
     var sel = window.getSelection();
     sel.removeAllRanges();
     var range = document.createRange();
diff --git a/layout/base/tests/bug512295-1.html b/layout/base/tests/bug512295-1.html
index 5f4816781087..f8346a1f4477 100644
--- a/layout/base/tests/bug512295-1.html
+++ b/layout/base/tests/bug512295-1.html
@@ -21,7 +21,7 @@ x
 
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm", window);
-  onSpellCheck(p.parentNode, function () {
+  maybeOnSpellCheck(p.parentNode, function () {
     sendKey('DOWN');  // now after "1"
     sendKey('DOWN');  // now make sure we get to the end
     sendKey('DOWN');  // now make sure we get to the end
diff --git a/layout/base/tests/bug512295-2-ref.html b/layout/base/tests/bug512295-2-ref.html
index 788ec44203a9..4ab9a99fd8c7 100644
--- a/layout/base/tests/bug512295-2-ref.html
+++ b/layout/base/tests/bug512295-2-ref.html
@@ -14,7 +14,7 @@ x
   div.focus();
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm", window);
-  onSpellCheck(div, function () {
+  maybeOnSpellCheck(div, function () {
     var sel = window.getSelection();
     sel.removeAllRanges();
     var range = document.createRange();
diff --git a/layout/base/tests/bug512295-2.html b/layout/base/tests/bug512295-2.html
index 063b99e0256e..d74af0fb3224 100644
--- a/layout/base/tests/bug512295-2.html
+++ b/layout/base/tests/bug512295-2.html
@@ -21,7 +21,7 @@ x
 
   SpecialPowers.Cu.import(
     "resource://testing-common/AsyncSpellCheckTestHelper.jsm", window);
-  onSpellCheck(p.parentNode, function () {
+  maybeOnSpellCheck(p.parentNode, function () {
     sendKey('DOWN');  // now after "1"
     sendKey('DOWN');  // now below the P element
     sendKey('UP');    // now before the "1"
diff --git a/layout/base/tests/bug923376-ref.html b/layout/base/tests/bug923376-ref.html
index 0ec11f7d0b57..b3f7dbdaa54c 100644
--- a/layout/base/tests/bug923376-ref.html
+++ b/layout/base/tests/bug923376-ref.html
@@ -5,7 +5,7 @@
 document.body.firstChild.focus();
 SpecialPowers.Cu.import(
   "resource://testing-common/AsyncSpellCheckTestHelper.jsm", window);
-onSpellCheck(document.body.firstChild, function() {
+maybeOnSpellCheck(document.body.firstChild, function() {
   document.documentElement.removeAttribute("class");
 });
 </script>
diff --git a/layout/base/tests/bug923376.html b/layout/base/tests/bug923376.html
index cab66a260b95..c516cea31d24 100644
--- a/layout/base/tests/bug923376.html
+++ b/layout/base/tests/bug923376.html
@@ -6,9 +6,9 @@ var div = document.body.firstChild;
 div.focus();
 SpecialPowers.Cu.import(
   "resource://testing-common/AsyncSpellCheckTestHelper.jsm", window);
-onSpellCheck(div, function() {
+maybeOnSpellCheck(div, function() {
   div.innerHTML = 'something missspelled<br>something elsed#';
-  onSpellCheck(div, function() {
+  maybeOnSpellCheck(div, function() {
     document.documentElement.removeAttribute("class");
   });
 });
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_dictionary_webextension.js b/toolkit/mozapps/extensions/test/xpcshell/test_dictionary_webextension.js
index bccb87a65a82..fe7afa321ea5 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_dictionary_webextension.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_dictionary_webextension.js
@@ -169,7 +169,7 @@ add_task(async function test_validation() {
 const WORD = "Flehgragh";
 
 add_task(async function test_registration() {
-  spellCheck.dictionary = "en-US";
+  spellCheck.dictionaries = ["en-US"];
 
   ok(!spellCheck.check(WORD), "Word should not pass check before add-on loads");