diff --git a/extensions/firefox/content/PdfJsDefaultPreferences.jsm b/extensions/firefox/content/PdfJsDefaultPreferences.sys.mjs
similarity index 85%
rename from extensions/firefox/content/PdfJsDefaultPreferences.jsm
rename to extensions/firefox/content/PdfJsDefaultPreferences.sys.mjs
index cc4b7b043..738adad80 100644
--- a/extensions/firefox/content/PdfJsDefaultPreferences.jsm
+++ b/extensions/firefox/content/PdfJsDefaultPreferences.sys.mjs
@@ -13,10 +13,6 @@
  * limitations under the License.
  */
 
-"use strict";
-
-var EXPORTED_SYMBOLS = ["PdfJsDefaultPreferences"];
-
-var PdfJsDefaultPreferences = Object.freeze(
+export const PdfJsDefaultPreferences = Object.freeze(
   PDFJSDev.eval("DEFAULT_PREFERENCES")
 );
diff --git a/gulpfile.js b/gulpfile.js
index 11894222f..1d32fdea8 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -434,12 +434,15 @@ function createSandboxExternal(defines) {
     saveComments: false,
     defines,
   };
-  return gulp.src("./src/pdf.sandbox.external.js").pipe(
-    transform("utf8", content => {
-      content = preprocessor2.preprocessPDFJSCode(ctx, content);
-      return `${licenseHeader}\n${content}`;
-    })
-  );
+  return gulp
+    .src("./src/pdf.sandbox.external.js")
+    .pipe(rename("pdf.sandbox.external.sys.mjs"))
+    .pipe(
+      transform("utf8", content => {
+        content = preprocessor2.preprocessPDFJSCode(ctx, content);
+        return `${licenseHeader}\n${content}`;
+      })
+    );
 }
 
 function createTemporaryScriptingBundle(defines, extraOptions = undefined) {
@@ -1381,7 +1384,7 @@ gulp.task(
           .pipe(gulp.dest(MOZCENTRAL_L10N_DIR)),
         gulp.src("LICENSE").pipe(gulp.dest(MOZCENTRAL_EXTENSION_DIR)),
         gulp
-          .src(FIREFOX_CONTENT_DIR + "PdfJsDefaultPreferences.jsm")
+          .src(FIREFOX_CONTENT_DIR + "PdfJsDefaultPreferences.sys.mjs")
           .pipe(transform("utf8", preprocessDefaultPreferences))
           .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR)),
       ]);
diff --git a/src/pdf.sandbox.external.js b/src/pdf.sandbox.external.js
index 68bb48a2d..ce77db1c6 100644
--- a/src/pdf.sandbox.external.js
+++ b/src/pdf.sandbox.external.js
@@ -16,7 +16,7 @@
 // In mozilla-central, this file is loaded as non-module script,
 // so it mustn't have any dependencies.
 
-class SandboxSupportBase {
+export class SandboxSupportBase {
   /**
    * @param {DOMWindow} - win
    */
@@ -181,10 +181,3 @@ class SandboxSupportBase {
     };
   }
 }
-
-if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
-  exports.SandboxSupportBase = SandboxSupportBase;
-} else {
-  /* eslint-disable-next-line no-unused-vars, no-var */
-  var EXPORTED_SYMBOLS = ["SandboxSupportBase"];
-}