Issue 5590 - [webextension] Custom filters in the options page aren't updated

ext/content.js

 "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.
-if (!("devtools" in chrome))
+// Note that Firefox processes API access lazily, so chrome.devtools will always
+// exist but will have undefined as its value on other pages.
+if (!chrome.devtools)
 {
   // Listen for messages from the background page.
   chrome.runtime.onMessage.addListener((message, sender, sendResponse) =>
   {
     return ext.onMessage._dispatch(message, {}, sendResponse).includes(true);
   });
 }
 
 (function()
 {
@@ skipping 17 matching lines @@
 
         if (!port.onDisconnect.hasListeners())
         {
           port.disconnect();
           port = null;
         }
       }
     }
   };
 }());