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

lib/content/elemHideEmulation.js

Index: lib/content/elemHideEmulation.js
===================================================================
--- a/lib/content/elemHideEmulation.js
+++ b/lib/content/elemHideEmulation.js
@@ -122,16 +122,30 @@
   }
   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) :
@@ -603,17 +617,17 @@
 
     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;