diff --git a/toolkit/components/translations/actors/TranslationsParent.sys.mjs b/toolkit/components/translations/actors/TranslationsParent.sys.mjs
index fec7927ee205..2a705c35ad29 100644
--- a/toolkit/components/translations/actors/TranslationsParent.sys.mjs
+++ b/toolkit/components/translations/actors/TranslationsParent.sys.mjs
@@ -62,6 +62,13 @@ export class TranslationsParent extends JSWindowActorParent {
   /** @type {RemoteSettingsClient | null} */
   #wasmRemoteClient = null;
 
+  /**
+   * The translation engine can be mocked for testing.
+   *
+   * @type {Array<{ fromLang: string, toLang: string }>}
+   */
+  static #mockedLanguagePairs = null;
+
   async receiveMessage({ name, data }) {
     switch (name) {
       case "Translations:GetBergamotWasmArrayBuffer": {
@@ -103,7 +110,10 @@ export class TranslationsParent extends JSWindowActorParent {
    */
   async #getSupportedLanguages() {
     const languages = new Set();
-    for (const { fromLang } of (await this.#getModelRecords()).values()) {
+    const languagePairs =
+      TranslationsParent.#mockedLanguagePairs ??
+      (await this.#getModelRecords()).values();
+    for (const { fromLang } of languagePairs) {
       languages.add(fromLang);
     }
 
@@ -416,6 +426,15 @@ export class TranslationsParent extends JSWindowActorParent {
 
     return results;
   }
+
+  /**
+   * For testing purposes, allow the Translations Engine to be mocked.
+   * @param {Array<{ fromLang: string, toLang: string }>} languagePairs
+   */
+  mock(languagePairs) {
+    lazy.console.log("Mocking language pairs", languagePairs);
+    TranslationsParent.#mockedLanguagePairs = languagePairs;
+  }
 }
 
 /**
diff --git a/toolkit/components/translations/tests/browser/browser_about_translations.js b/toolkit/components/translations/tests/browser/browser_about_translations.js
index 6f903ab0ee26..4fdf16c50595 100644
--- a/toolkit/components/translations/tests/browser/browser_about_translations.js
+++ b/toolkit/components/translations/tests/browser/browser_about_translations.js
@@ -55,3 +55,94 @@ add_task(async function test_about_translations_disabled() {
     },
   });
 });
+
+add_task(async function test_about_translations_dropdowns() {
+  const { appLocaleAsBCP47 } = Services.locale;
+  if (!appLocaleAsBCP47.startsWith("en")) {
+    console.warn(
+      "This test assumes to be running in an 'en' app locale, however the app locale " +
+        `is set to ${appLocaleAsBCP47}. Skipping the test.`
+    );
+    ok(true, "Skipping test.");
+    return;
+  }
+
+  await openAboutTranslations({
+    languagePairs: [
+      { fromLang: "en", toLang: "es" },
+      { fromLang: "es", toLang: "en" },
+      // This is not a bi-directional translation.
+      { fromLang: "is", toLang: "en" },
+    ],
+    runInPage: async ({ selectors }) => {
+      const { document } = content;
+
+      /**
+       * Some languages can be marked as hidden in the dropbdown. This function
+       * asserts the configuration of the options.
+       *
+       * @param {object} args
+       * @param {string} args.message
+       * @param {HTMLSelectElement} args.select
+       * @param {string[]} args.availableOptions
+       * @param {string} args.selectedValue
+       */
+      function assertOptions({
+        message,
+        select,
+        availableOptions,
+        selectedValue,
+      }) {
+        const options = [...select.options]
+          .filter(option => !option.hidden)
+          .map(option => option.value);
+
+        info(message);
+        Assert.deepEqual(
+          options,
+          availableOptions,
+          "The available options match."
+        );
+        is(selectedValue, select.value, "The selected value matches.");
+      }
+
+      /** @type {HTMLSelectElement} */
+      const fromSelect = document.querySelector(selectors.fromLanguageSelect);
+      /** @type {HTMLSelectElement} */
+      const toSelect = document.querySelector(selectors.toLanguageSelect);
+
+      assertOptions({
+        message: "From languages have English already selected.",
+        select: fromSelect,
+        availableOptions: ["", "en", "is", "es"],
+        selectedValue: "en",
+      });
+
+      assertOptions({
+        message:
+          'The "to" options do not have "en" in the list, and nothing is selected.',
+        select: toSelect,
+        availableOptions: ["", "is", "es"],
+        selectedValue: "",
+      });
+
+      info('Switch the "to" language to Spanish.');
+      toSelect.value = "es";
+      toSelect.dispatchEvent(new Event("input"));
+
+      assertOptions({
+        message: 'The "from" languages no longer suggest Spanish.',
+        select: fromSelect,
+        availableOptions: ["", "en", "is"],
+        selectedValue: "en",
+      });
+
+      assertOptions({
+        message: 'The "to" options remain the same.',
+        select: toSelect,
+        availableOptions: ["", "is", "es"],
+        selectedValue: "es",
+      });
+    },
+  });
+});
diff --git a/toolkit/components/translations/tests/browser/head.js b/toolkit/components/translations/tests/browser/head.js
index 020c5219475b..cf5f8a1adc5d 100644
--- a/toolkit/components/translations/tests/browser/head.js
+++ b/toolkit/components/translations/tests/browser/head.js
@@ -22,8 +22,16 @@
  *
  * @param {boolean} [options.disabled]
  * Disable the panel through a pref.
+ *
+ * @param {Array<{ fromLang: string, toLang: string}>} options.languagePairs
+ * The translation languages pairs to mock for the test.
  */
-async function openAboutTranslations({ dataForContent, disabled, runInPage }) {
+async function openAboutTranslations({
+  dataForContent,
+  disabled,
+  runInPage,
+  languagePairs,
+}) {
   await SpecialPowers.pushPrefEnv({
     set: [
       // Enabled by default.
@@ -44,18 +52,38 @@ async function openAboutTranslations({ dataForContent, disabled, runInPage }) {
     translationResultBlank: "#translation-to-blank",
   };
 
+  // Start the tab at about:blank.
   let tab = await BrowserTestUtils.openNewForegroundTab(
     gBrowser,
-    "about:translations",
+    "about:blank",
     true // waitForLoad
   );
 
+  // Before loading about:translations, handle any mocking of the actor.
+  if (languagePairs) {
+    const translations = tab.linkedBrowser.browsingContext.currentWindowGlobal.getActor(
+      "Translations"
+    );
+    translations.mock(languagePairs);
+  }
+
+  // Now load the about:translations page, since the actor could be mocked.
+  BrowserTestUtils.loadURIString(tab.linkedBrowser, "about:translations");
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+
   await ContentTask.spawn(
     tab.linkedBrowser,
     { dataForContent, selectors },
     runInPage
   );
 
+  if (languagePairs) {
+    // Revert the mock.
+    const translations = tab.linkedBrowser.browsingContext.currentWindowGlobal.getActor(
+      "Translations"
+    );
+    translations.mock(null);
+  }
   BrowserTestUtils.removeTab(tab);
   await SpecialPowers.popPrefEnv();
 }