diff --git a/uriloader/exthandler/HandlerService.js b/uriloader/exthandler/HandlerService.js
index c7263d7aa9da..54ce5eecc4ce 100644
--- a/uriloader/exthandler/HandlerService.js
+++ b/uriloader/exthandler/HandlerService.js
@@ -158,6 +158,11 @@ HandlerService.prototype = {
     // don't care what's already in the file, we just want to do the
     // equivalent of appending into the database. So let's just go do that:
     for (let scheme of Object.keys(localeHandlers.schemes)) {
+      if (scheme == "mailto" && AppConstants.MOZ_APP_NAME == "thunderbird") {
+        // Thunderbird IS a mailto handler, it doesn't need handlers added.
+        continue;
+      }
+
       let existingSchemeInfo = this._store.data.schemes[scheme];
       if (!existingSchemeInfo) {
         // Haven't seen this scheme before. Default to asking which app the
diff --git a/uriloader/exthandler/tests/unit/test_handlerService.js b/uriloader/exthandler/tests/unit/test_handlerService.js
index c6450219ed72..4edb4900563b 100644
--- a/uriloader/exthandler/tests/unit/test_handlerService.js
+++ b/uriloader/exthandler/tests/unit/test_handlerService.js
@@ -155,7 +155,11 @@ function run_test() {
   // OS default exists, injected default exists, explicit warning pref: false
   prefSvc.setBoolPref(kExternalWarningPrefPrefix + "mailto", false);
   protoInfo = protoSvc.getProtocolHandlerInfo("mailto");
-  Assert.equal(1, protoInfo.possibleApplicationHandlers.length);
+  if (AppConstants.MOZ_APP_NAME == "thunderbird") {
+    Assert.equal(0, protoInfo.possibleApplicationHandlers.length);
+  } else {
+    Assert.equal(1, protoInfo.possibleApplicationHandlers.length);
+  }
 
   // Win7+ or Linux's GIO might not have a default mailto: handler
   if (noMailto) {
@@ -167,16 +171,19 @@ function run_test() {
   // OS default exists, injected default exists, explicit warning pref: true
   prefSvc.setBoolPref(kExternalWarningPrefPrefix + "mailto", true);
   protoInfo = protoSvc.getProtocolHandlerInfo("mailto");
-  Assert.equal(1, protoInfo.possibleApplicationHandlers.length);
-  // Win7+ or Linux's GIO may have no default mailto: handler, so we'd ask
-  // anyway. Otherwise, the default handlers will not have stored preferred
-  // actions etc., so re-requesting them after the warning pref has changed
-  // will use the updated pref value. So both when we have and do not have
-  // a default mailto: handler, we'll ask:
-  Assert.ok(protoInfo.alwaysAskBeforeHandling);
-  // As soon as anyone actually stores updated defaults into the profile
-  // database, that default will stop tracking the warning pref.
-
+  if (AppConstants.MOZ_APP_NAME == "thunderbird") {
+    Assert.equal(0, protoInfo.possibleApplicationHandlers.length);
+  } else {
+    Assert.equal(1, protoInfo.possibleApplicationHandlers.length);
+    // Win7+ or Linux's GIO may have no default mailto: handler, so we'd ask
+    // anyway. Otherwise, the default handlers will not have stored preferred
+    // actions etc., so re-requesting them after the warning pref has changed
+    // will use the updated pref value. So both when we have and do not have
+    // a default mailto: handler, we'll ask:
+    Assert.ok(protoInfo.alwaysAskBeforeHandling);
+    // As soon as anyone actually stores updated defaults into the profile
+    // database, that default will stop tracking the warning pref.
+  }
   // Now set the value stored in RDF to true, and the pref to false, to make
   // sure we still get the right value. (Basically, same thing as above but
   // with the values reversed.)
@@ -184,8 +191,12 @@ function run_test() {
   protoInfo.alwaysAskBeforeHandling = true;
   handlerSvc.store(protoInfo);
   protoInfo = protoSvc.getProtocolHandlerInfo("mailto");
-  Assert.equal(1, protoInfo.possibleApplicationHandlers.length);
-  Assert.ok(protoInfo.alwaysAskBeforeHandling);
+  if (AppConstants.MOZ_APP_NAME == "thunderbird") {
+    Assert.equal(0, protoInfo.possibleApplicationHandlers.length);
+  } else {
+    Assert.equal(1, protoInfo.possibleApplicationHandlers.length);
+    Assert.ok(protoInfo.alwaysAskBeforeHandling);
+  }
 
   //* *************************************************************************//
   // Test Round-Trip Data Integrity
diff --git a/uriloader/exthandler/tests/unit/test_handlerService_store.js b/uriloader/exthandler/tests/unit/test_handlerService_store.js
index f9d6e8d90d43..d3f4e2818d00 100644
--- a/uriloader/exthandler/tests/unit/test_handlerService_store.js
+++ b/uriloader/exthandler/tests/unit/test_handlerService_store.js
@@ -595,43 +595,49 @@ function assertAllHandlerInfosMatchDefaultHandlers() {
 /**
  * Tests the default protocol handlers imported from the locale-specific data.
  */
-add_task(async function test_default_protocol_handlers() {
-  if (
-    !Services.prefs.getPrefType("gecko.handlerService.defaultHandlersVersion")
-  ) {
-    info("This platform or locale does not have default handlers.");
-    return;
+add_task(
+  { skip_if: () => AppConstants.MOZ_APP_NAME == "thunderbird" },
+  async function test_default_protocol_handlers() {
+    if (
+      !Services.prefs.getPrefType("gecko.handlerService.defaultHandlersVersion")
+    ) {
+      info("This platform or locale does not have default handlers.");
+      return;
+    }
+
+    // This will inject the default protocol handlers for the current locale.
+    await deleteHandlerStore();
+
+    await assertAllHandlerInfosMatchDefaultHandlers();
   }
-
-  // This will inject the default protocol handlers for the current locale.
-  await deleteHandlerStore();
-
-  await assertAllHandlerInfosMatchDefaultHandlers();
-});
+);
 
 /**
  * Tests that the default protocol handlers are not imported again from the
  * locale-specific data if they already exist.
  */
-add_task(async function test_default_protocol_handlers_no_duplicates() {
-  if (
-    !Services.prefs.getPrefType("gecko.handlerService.defaultHandlersVersion")
-  ) {
-    info("This platform or locale does not have default handlers.");
-    return;
+add_task(
+  { skip_if: () => AppConstants.MOZ_APP_NAME == "thunderbird" },
+  async function test_default_protocol_handlers_no_duplicates() {
+    if (
+      !Services.prefs.getPrefType("gecko.handlerService.defaultHandlersVersion")
+    ) {
+      info("This platform or locale does not have default handlers.");
+      return;
+    }
+
+    // This will inject the default protocol handlers for the current locale.
+    await deleteHandlerStore();
+
+    // Clear the preference to force injecting again.
+    Services.prefs.clearUserPref("gecko.handlerService.defaultHandlersVersion");
+
+    await unloadHandlerStore();
+
+    // There should be no duplicate handlers in the protocols.
+    assertAllHandlerInfosMatchDefaultHandlers();
   }
-
-  // This will inject the default protocol handlers for the current locale.
-  await deleteHandlerStore();
-
-  // Clear the preference to force injecting again.
-  Services.prefs.clearUserPref("gecko.handlerService.defaultHandlersVersion");
-
-  await unloadHandlerStore();
-
-  // There should be no duplicate handlers in the protocols.
-  assertAllHandlerInfosMatchDefaultHandlers();
-});
+);
 
 /**
  * Ensures forward compatibility by checking that the "store" method preserves