diff --git a/.eslintrc b/.eslintrc
index a6c315c2d..1c5c97790 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -31,6 +31,7 @@
 
   "rules": {
     // Plugins
+    "import/extensions": ["error", "always", { "ignorePackages": true, }],
     "import/no-unresolved": "error",
     "mozilla/avoid-removeChild": "error",
     "mozilla/use-includes-instead-of-indexOf": "error",
diff --git a/src/shared/util.js b/src/shared/util.js
index 0584c6630..0777550bf 100644
--- a/src/shared/util.js
+++ b/src/shared/util.js
@@ -14,7 +14,7 @@
  */
 /* eslint no-var: error */
 
-import "./compatibility";
+import "./compatibility.js";
 
 const IDENTITY_MATRIX = [1, 0, 0, 1, 0, 0];
 const FONT_IDENTITY_MATRIX = [0.001, 0, 0, 0.001, 0, 0];
diff --git a/web/firefoxcom.js b/web/firefoxcom.js
index 83db636ce..48703f83b 100644
--- a/web/firefoxcom.js
+++ b/web/firefoxcom.js
@@ -13,7 +13,7 @@
  * limitations under the License.
  */
 
-import "../extensions/firefox/tools/l10n";
+import "../extensions/firefox/tools/l10n.js";
 import { createObjectURL, PDFDataRangeTransport, shadow } from "pdfjs-lib";
 import { BasePreferences } from "./preferences.js";
 import { DEFAULT_SCALE_VALUE } from "./ui_utils.js";
diff --git a/web/genericl10n.js b/web/genericl10n.js
index dae376505..d260e9be2 100644
--- a/web/genericl10n.js
+++ b/web/genericl10n.js
@@ -13,7 +13,7 @@
  * limitations under the License.
  */
 
-import "../external/webL10n/l10n";
+import "../external/webL10n/l10n.js";
 
 const webL10n = document.webL10n;