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 +406,51 @@
     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)
+      {
+        try
+        {
+          // Explicitly ignore third-party stylesheets to ensure consistent behavior

[kzar, 2017/11/03 10:20:50]

Nit: This line is too long, did you check the code lints?

+          // between Firefox and Chrome.
+          if (!this.isSameOrigin(stylesheet))
+            continue;
+
+          let rules = stylesheet.cssRules;
+          if (!rules)
+            continue;
 
-      let rules = stylesheet.cssRules;
-      if (!rules)
-        continue;
+          for (let rule of rules)
+          {
+            if (rule.type != rule.STYLE_RULE)
+              continue;
 
-      for (let rule of rules)
-      {
-        if (rule.type != rule.STYLE_RULE)
-          continue;
-
-        cssStyles.push(stringifyStyle(rule));
+            cssStyles.push(stringifyStyle(rule));
+          }
+        }
+        catch (e)
+        {
+          // On Firefox we seem to get a InvalidAccessError when

[kzar, 2017/11/03 10:20:50]

Nit: "...get an...".

+          // accessing the stylesheet under some circumstances.
+        }
       }
     }
+    catch (e)
+    {
+      // On Firefox we seem to get a InvalidAccessError when

[kzar, 2017/11/03 10:20:50]

Maybe combine these two identical comments and add some more details like the
Firefox version or a Mozilla issue number?

+      // enumerating the stylesheet under some circumstances.
+    }
 
     let patterns = this.patterns.slice();
     let pattern = null;
     let generator = null;
 
     let processPatterns = () =>
     {
       let cycleStart = performance.now();