Issue 6424 - Use internal symbol in ext namespace

ext/content.js

Index: ext/content.js
===================================================================
--- a/ext/content.js
+++ b/ext/content.js
@@ -1,22 +1,26 @@
+/* global internal */
+
 "use strict";
 
 // Firefox 55 erroneously sends messages from the content script to the
 // devtools panel:
 // https://bugzilla.mozilla.org/show_bug.cgi?id=1383310
 // As a workaround, listen for messages only if this isn't the devtools panel.
 // Note that Firefox processes API access lazily, so browser.devtools will
 // always exist but will have undefined as its value on other pages.
 if (!browser.devtools)
 {
   // Listen for messages from the background page.
   browser.runtime.onMessage.addListener((message, sender, sendResponse) =>
   {
-    return ext.onMessage._dispatch(message, {}, sendResponse).includes(true);
+    return ext.onMessage[internal].dispatch(
+      message, {}, sendResponse
+    ).includes(true);
   });
 }
 
 {
   let port = null;
 
   ext.onExtensionUnloaded = {
     addListener(listener)