Issue 6594 - Ignore error when accessing another extension's CSS rules

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 109 matching lines @@
     let priority = rule.style.getPropertyPriority(property);
     styles.push(`${property}: ${value}${priority ? " !" + priority : ""};`);
   }
   styles.sort();
   return {
     style: styles.join(" "),
     subSelectors: splitSelector(rule.selectorText)
   };
 }
 
+function getCSSRules(stylesheet)
+{
+  try
+  {
+    return stylesheet.cssRules;
+  }
+  catch (e)
+  {
+    // On Firefox, a style sheet injected by another extension has the same
+    // origin, but accessing the cssRules property is still an error.
+    // See #6594.
+  }
+}
+
 let scopeSupported = null;
 
 function tryQuerySelector(subtree, selector, all)
 {
   let elements = null;
   try
   {
     elements = all ? subtree.querySelectorAll(selector) :
       subtree.querySelector(selector);
     scopeSupported = true;
@@ skipping 461 matching lines @@
     if (mutations && patterns.some(pattern => pattern.dependsOnStylesAndDOM))
       stylesheets = this.document.styleSheets;
 
     for (let stylesheet of stylesheets || [])
     {
       // Explicitly ignore third-party stylesheets to ensure consistent behavior
       // between Firefox and Chrome.
       if (!this.isSameOrigin(stylesheet))
         continue;
 
-      let rules = stylesheet.cssRules;
+      let rules = getCSSRules(stylesheet);
       if (!rules)
         continue;
 
       for (let rule of rules)
       {
         if (rule.type != rule.STYLE_RULE)
           continue;
 
         cssStyles.push(stringifyStyle(rule));
       }
@@ skipping 187 matching lines @@
           characterData: shouldObserveCharacterData(this.patterns),
           subtree: true
         }
       );
       this.document.addEventListener("load", this.onLoad.bind(this), true);
     }
   }
 };
 
 exports.ElemHideEmulation = ElemHideEmulation;