Issue 5956 - Catch exception when enumerating stylesheets

lib/content/elemHideEmulation.js

Index: lib/content/elemHideEmulation.js
===================================================================
--- a/lib/content/elemHideEmulation.js
+++ b/lib/content/elemHideEmulation.js
@@ -406,35 +406,44 @@
     let elementFilters = [];
 
     let cssStyles = [];
 
     let stylesheetOnlyChange = !!stylesheets;
     if (!stylesheets)
       stylesheets = this.document.styleSheets;
 
-    for (let stylesheet of stylesheets)
+    try
     {
-      // Explicitly ignore third-party stylesheets to ensure consistent behavior
-      // between Firefox and Chrome.
-      if (!this.isSameOrigin(stylesheet))
-        continue;
+      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;
-      if (!rules)
-        continue;
-
-      for (let rule of rules)
-      {
-        if (rule.type != rule.STYLE_RULE)
+        let rules = stylesheet.cssRules;
+        if (!rules)
           continue;
 
-        cssStyles.push(stringifyStyle(rule));
+        for (let rule of rules)
+        {
+          if (rule.type != rule.STYLE_RULE)
+            continue;
+
+          cssStyles.push(stringifyStyle(rule));
+        }
       }
     }
+    catch (e)
+    {
+      // Make sure to not interrupt.
+      // On Firefox we seem to get a InvalidAccessError when
+      // enumerating the stylesheets.
+    }
 
     let patterns = this.patterns.slice();
     let pattern = null;
     let generator = null;
 
     let processPatterns = () =>
     {
       let cycleStart = performance.now();