Issue 5956 - Fix intermittent element hiding emulation failures in Firefox

lib/content/elemHideEmulation.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 533 matching lines @@
       this._scheduledProcessing = {stylesheets};
       setTimeout(() =>
       {
         let newStylesheets = this._scheduledProcessing.stylesheets;
         this._filteringInProgress = true;
         this._scheduledProcessing = null;
         this._addSelectors(newStylesheets, completion);
       },
       MIN_INVOCATION_INTERVAL - (performance.now() - this._lastInvocation));
     }
+    else if (this.document.readyState == "loading")
+    {
+      this._scheduledProcessing = {stylesheets};
+      let handler = () =>
+      {
+        document.removeEventListener("DOMContentLoaded", handler);
+        let newStylesheets = this._scheduledProcessing.stylesheets;
+        this._filteringInProgress = true;
+        this._scheduledProcessing = null;
+        this._addSelectors(newStylesheets, completion);
+      };
+      document.addEventListener("DOMContentLoaded", handler);
+    }
     else
     {
       this._filteringInProgress = true;
       this._addSelectors(stylesheets, completion);
     }
   },
 
   onLoad(event)
   {
     let stylesheet = event.target.sheet;
@@ skipping 27 matching lines @@
           characterData: true,
           subtree: true
         }
       );
       this.document.addEventListener("load", this.onLoad.bind(this), true);
     }
   }
 };
 
 exports.ElemHideEmulation = ElemHideEmulation;