Issue 6955 - Avoid making copies of common selector list

lib/elemHide.js

Index: lib/elemHide.js
===================================================================
--- a/lib/elemHide.js
+++ b/lib/elemHide.js
@@ -81,17 +81,17 @@
 
 /**
  * Returns a list of selectors that apply on each website unconditionally.
  * @returns {string[]}
  */
 function getUnconditionalSelectors()
 {
   if (!unconditionalSelectors)
-    unconditionalSelectors = [...filterBySelector.keys()];
+    unconditionalSelectors = Object.freeze([...filterBySelector.keys()]);
 
   return unconditionalSelectors;
 }
 
 ElemHideExceptions.on("added", ({selector}) =>
 {
   // If this is the first exception for a previously unconditionally applied
   // element hiding selector we need to take care to update the lookups.
@@ -231,13 +231,18 @@
       if (currentDomain == "")
         break;
 
       let nextDot = currentDomain.indexOf(".");
       currentDomain = nextDot == -1 ? "" : currentDomain.substr(nextDot + 1);
     }
 
     if (!specificOnly)
-      selectors = getUnconditionalSelectors().concat(selectors);
+    {
+      // Avoid making a copy if there are no domain-specific selectors.
+      selectors = selectors.length > 0 ?
+                    getUnconditionalSelectors().concat(selectors) :
+                    getUnconditionalSelectors();
+    }
 
     return selectors;
   }
 };