diff --git a/browser/base/content/browser-charsetmenu.inc b/browser/base/content/browser-charsetmenu.inc
index dca869a92e5a..628de1341fea 100644
--- a/browser/base/content/browser-charsetmenu.inc
+++ b/browser/base/content/browser-charsetmenu.inc
@@ -11,9 +11,9 @@
 #endif
     oncommand="MultiplexHandler(event)"
 #ifdef OMIT_ACCESSKEYS
-    onpopupshowing="CharsetMenu.build(event);"
+#expand    onpopupshowing="CharsetMenu.build(event, '__ID_PREFIX__');"
 #else
-    onpopupshowing="CharsetMenu.build(event, true);"
+#expand    onpopupshowing="CharsetMenu.build(event, '__ID_PREFIX__', true);"
 #endif
     onpopupshown="UpdateMenus(event);">
   <menupopup>
diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index c9d226d18ff2..57a941970b0b 100644
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -5087,7 +5087,7 @@ function MultiplexHandler(event)
         SelectDetector(event, false);
     } else if (name == 'charsetGroup') {
         var charset = node.getAttribute('id');
-        charset = charset.substring('charset.'.length, charset.length)
+        charset = charset.substring(charset.indexOf('charset.') + 'charset.'.length);
         BrowserSetForcedCharacterSet(charset);
     } else if (name == 'charsetCustomize') {
         //do nothing - please remove this else statement, once the charset prefs moves to the pref window
@@ -5100,7 +5100,7 @@ function MultiplexHandler(event)
 function SelectDetector(event, doReload)
 {
     var uri =  event.target.getAttribute("id");
-    var prefvalue = uri.substring('chardet.'.length, uri.length);
+    var prefvalue = uri.substring(uri.indexOf('chardet.') + 'chardet.'.length);
     if ("off" == prefvalue) { // "off" is special value to turn off the detectors
         prefvalue = "";
     }
diff --git a/browser/modules/CharsetMenu.jsm b/browser/modules/CharsetMenu.jsm
index ed6a10ee4e87..75c45953dfca 100644
--- a/browser/modules/CharsetMenu.jsm
+++ b/browser/modules/CharsetMenu.jsm
@@ -82,7 +82,7 @@ const kPinned = [
 ];
 
 this.CharsetMenu = Object.freeze({
-  build: function BuildCharsetMenu(event, showAccessKeys) {
+  build: function BuildCharsetMenu(event, idPrefix="", showAccessKeys=false) {
     let parent = event.target;
     if (parent.lastChild.localName != "menuseparator") {
       // Detector menu or charset menu already built
@@ -108,7 +108,7 @@ this.CharsetMenu = Object.freeze({
           // Some items intentionally don't have an accesskey
         }
       }
-      menuItem.setAttribute("id", "charset." + encoding);
+      menuItem.setAttribute("id", idPrefix + "charset." + encoding);
       return menuItem;
     }