diff --git a/extensions/firefox/content/streams_polyfill.js b/extensions/firefox/content/streams_polyfill.js
new file mode 100644
index 000000000..73ef7c75d
--- /dev/null
+++ b/extensions/firefox/content/streams_polyfill.js
@@ -0,0 +1,19 @@
+/* Copyright 2018 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+if (typeof ReadableStream === "undefined") {
+  require("../../../src/shared/global_scope").ReadableStream =
+    require("../../../external/streams/streams-lib").ReadableStream;
+}
diff --git a/gulpfile.js b/gulpfile.js
index 685a4d575..6604ef23b 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -766,6 +766,16 @@ function preprocessDefaultPreferences(content) {
           content + '\n');
 }
 
+function createMozcentralStreamsPolyfillBundle(defines) {
+  var streamsPolyfillOutputName = 'streams_polyfill.js';
+
+  var streamsPolyfillFileConfig = createWebpackConfig(defines, {
+    filename: streamsPolyfillOutputName,
+  });
+  return gulp.src('./extensions/firefox/content/streams_polyfill.js')
+             .pipe(webpack2Stream(streamsPolyfillFileConfig));
+}
+
 gulp.task('mozcentral-pre', gulp.series('buildnumber', 'locale', function () {
   console.log();
   console.log('### Building mozilla-central extension');
@@ -787,6 +797,8 @@ gulp.task('mozcentral-pre', gulp.series('buildnumber', 'locale', function () {
   return merge([
     createBundle(defines).pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + 'build')),
     createWebBundle(defines).pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + 'web')),
+    createMozcentralStreamsPolyfillBundle(defines)
+        .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + 'build')),
     gulp.src(COMMON_WEB_FILES, { base: 'web/', })
         .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + 'web')),
     gulp.src(['external/bcmaps/*.bcmap', 'external/bcmaps/LICENSE'],
diff --git a/src/pdf.worker.js b/src/pdf.worker.js
index 7306f17d2..afce1daeb 100644
--- a/src/pdf.worker.js
+++ b/src/pdf.worker.js
@@ -12,13 +12,17 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* eslint-disable no-unused-vars */
+/* eslint-disable no-restricted-globals, no-unused-vars */
 
 'use strict';
 
-var pdfjsVersion = PDFJSDev.eval('BUNDLE_VERSION');
-var pdfjsBuild = PDFJSDev.eval('BUNDLE_BUILD');
+if (PDFJSDev.test('MOZCENTRAL') && typeof ReadableStream === 'undefined') {
+  importScripts('./streams_polyfill.js');
+}
 
-var pdfjsCoreWorker = require('./core/worker.js');
+const pdfjsVersion = PDFJSDev.eval('BUNDLE_VERSION');
+const pdfjsBuild = PDFJSDev.eval('BUNDLE_BUILD');
+
+const pdfjsCoreWorker = require('./core/worker.js');
 
 exports.WorkerMessageHandler = pdfjsCoreWorker.WorkerMessageHandler;
diff --git a/src/shared/streams_polyfill.js b/src/shared/streams_polyfill.js
index 9dbaaf5c8..fbe70c092 100644
--- a/src/shared/streams_polyfill.js
+++ b/src/shared/streams_polyfill.js
@@ -14,25 +14,32 @@
  */
 /* eslint-disable no-restricted-globals */
 
-let isReadableStreamSupported = false;
-if (typeof ReadableStream !== 'undefined') {
-  // MS Edge may say it has ReadableStream but they are not up to spec yet.
-  try {
-    // eslint-disable-next-line no-new
-    new ReadableStream({
-      start(controller) {
-        controller.close();
-      },
-    });
-    isReadableStreamSupported = true;
-  } catch (e) {
-    // The ReadableStream constructor cannot be used.
-  }
-}
-if (isReadableStreamSupported) {
+if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('MOZCENTRAL')) {
+  // On the main-thread the `streams_polyfill.js` file is loaded using a
+  // <script> tag; see `web/viewer-snippet-firefox-extension.html`.
+  // On the worker-thread the `streams_polyfill.js` file is (conditionally)
+  // loaded using `importScripts`; see `src/pdf.worker.js`.
   exports.ReadableStream = ReadableStream;
 } else {
-  if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('IMAGE_DECODERS')) {
+  let isReadableStreamSupported = false;
+  if (typeof ReadableStream !== 'undefined') {
+    // MS Edge may say it has ReadableStream but they are not up to spec yet.
+    try {
+      // eslint-disable-next-line no-new
+      new ReadableStream({
+        start(controller) {
+          controller.close();
+        },
+      });
+      isReadableStreamSupported = true;
+    } catch (e) {
+      // The ReadableStream constructor cannot be used.
+    }
+  }
+  if (isReadableStreamSupported) {
+    exports.ReadableStream = ReadableStream;
+  } else if (typeof PDFJSDev !== 'undefined' &&
+             PDFJSDev.test('IMAGE_DECODERS')) {
     class DummyReadableStream {
       constructor() {
         throw new Error('The current image decoders are synchronous, ' +
diff --git a/src/shared/url_polyfill.js b/src/shared/url_polyfill.js
index b16ba9b78..4b8039e4d 100644
--- a/src/shared/url_polyfill.js
+++ b/src/shared/url_polyfill.js
@@ -32,32 +32,31 @@ if (typeof PDFJSDev !== 'undefined' && !PDFJSDev.test('GENERIC')) {
 
   if (isURLSupported) {
     exports.URL = URL;
-  } else {
-    if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('IMAGE_DECODERS')) {
-      class DummyURL {
-        constructor() {
-          throw new Error('The current image decoders doesn\'t utilize the ' +
-                          '`URL` constructor, hence it shouldn\'t need to be ' +
-                          'polyfilled for the IMAGE_DECODERS build target.');
-        }
+  } else if (typeof PDFJSDev !== 'undefined' &&
+             PDFJSDev.test('IMAGE_DECODERS')) {
+    class DummyURL {
+      constructor() {
+        throw new Error('The current image decoders doesn\'t utilize the ' +
+                        '`URL` constructor, hence it shouldn\'t need to be ' +
+                        'polyfilled for the IMAGE_DECODERS build target.');
       }
-      exports.URL = DummyURL;
-    } else {
-      const PolyfillURL = require('../../external/url/url-lib').URL;
-
-      // Attempt to copy over the static methods.
-      const OriginalURL = require('./global_scope').URL;
-      if (OriginalURL) {
-        PolyfillURL.createObjectURL = function(blob) {
-          // IE extension allows a second optional options argument, see
-          // http://msdn.microsoft.com/en-us/library/ie/hh772302(v=vs.85).aspx
-          return OriginalURL.createObjectURL.apply(OriginalURL, arguments);
-        };
-        PolyfillURL.revokeObjectURL = function(url) {
-          OriginalURL.revokeObjectURL(url);
-        };
-      }
-      exports.URL = PolyfillURL;
     }
+    exports.URL = DummyURL;
+  } else {
+    const PolyfillURL = require('../../external/url/url-lib').URL;
+
+    // Attempt to copy over the static methods.
+    const OriginalURL = require('./global_scope').URL;
+    if (OriginalURL) {
+      PolyfillURL.createObjectURL = function(blob) {
+        // IE extension allows a second optional options argument, see
+        // http://msdn.microsoft.com/en-us/library/ie/hh772302(v=vs.85).aspx
+        return OriginalURL.createObjectURL.apply(OriginalURL, arguments);
+      };
+      PolyfillURL.revokeObjectURL = function(url) {
+        OriginalURL.revokeObjectURL(url);
+      };
+    }
+    exports.URL = PolyfillURL;
   }
 }
diff --git a/web/viewer-snippet-firefox-extension.html b/web/viewer-snippet-firefox-extension.html
index b153febfa..f8e2596db 100644
--- a/web/viewer-snippet-firefox-extension.html
+++ b/web/viewer-snippet-firefox-extension.html
@@ -1,3 +1,10 @@
 <!-- This snippet is used in the Firefox extension (included from viewer.html) -->
 <base href="resource://pdf.js/web/">
+<script>
+  (function() {
+    if (typeof ReadableStream === "undefined") {
+      document.write("<script src=\"../build/streams_polyfill.js\"><\/script>");
+    }
+  })();
+</script>
 <script src="../build/pdf.js"></script>