diff --git a/extensions/firefox/.eslintrc b/extensions/firefox/.eslintrc
index 7c4e6e682..3074a764a 100644
--- a/extensions/firefox/.eslintrc
+++ b/extensions/firefox/.eslintrc
@@ -7,7 +7,7 @@
   ],
 
   "parserOptions": {
-    "sourceType": "script",
+    "sourceType": "module",
   },
 
   "plugins": [
diff --git a/external/importL10n/locales.js b/external/importL10n/locales.mjs
similarity index 95%
rename from external/importL10n/locales.js
rename to external/importL10n/locales.mjs
index 0e16f3843..04d818f88 100644
--- a/external/importL10n/locales.js
+++ b/external/importL10n/locales.mjs
@@ -13,11 +13,9 @@
  * limitations under the License.
  */
 
-"use strict";
-
-const fs = require("fs");
-const https = require("https");
-const path = require("path");
+import fs from "fs";
+import https from "https";
+import path from "path";
 
 // Fetches all languages that have an *active* translation in mozilla-central.
 // This is used in gulpfile.js for the `importl10n` command.
@@ -112,7 +110,7 @@ function downloadLanguageFiles(root, langCode) {
   });
 }
 
-async function downloadL10n(root, callback) {
+async function downloadL10n(root) {
   const langCodes = await downloadLanguageCodes();
 
   for (const langCode of langCodes) {
@@ -142,10 +140,6 @@ async function downloadL10n(root, callback) {
         "\n"
     );
   }
-
-  if (callback) {
-    callback();
-  }
 }
 
-exports.downloadL10n = downloadL10n;
+export { downloadL10n };
diff --git a/gulpfile.js b/gulpfile.js
index 20b37a092..8dfff41b4 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -1880,7 +1880,7 @@ gulp.task("lint", function (done) {
   const esLintOptions = [
     "node_modules/eslint/bin/eslint",
     "--ext",
-    ".js,.jsm,.json",
+    ".js,.jsm,.mjs,.json",
     ".",
     "--report-unused-disable-directives",
   ];
@@ -2004,8 +2004,8 @@ gulp.task("clean", function (done) {
   rimraf(BUILD_DIR, done);
 });
 
-gulp.task("importl10n", function (done) {
-  const locales = require("./external/importL10n/locales.js");
+gulp.task("importl10n", async function () {
+  const { downloadL10n } = await import("./external/importL10n/locales.mjs");
 
   console.log();
   console.log("### Importing translations from mozilla-central");
@@ -2013,7 +2013,7 @@ gulp.task("importl10n", function (done) {
   if (!fs.existsSync(L10N_DIR)) {
     fs.mkdirSync(L10N_DIR);
   }
-  locales.downloadL10n(L10N_DIR, done);
+  await downloadL10n(L10N_DIR);
 });
 
 function ghPagesPrepare() {