Issue 6967 - Hold on to only non-standard style sheets

lib/contentFiltering.js

Index: lib/contentFiltering.js
===================================================================
--- a/lib/contentFiltering.js
+++ b/lib/contentFiltering.js
@@ -156,17 +156,22 @@
     return false;
 
   // Sometimes the old and new style sheets can be exactly the same. In such a
   // case, do not remove the "old" style sheet, because it is in fact the new
   // style sheet now.
   if (oldStyleSheet && oldStyleSheet != styleSheet)
     removeStyleSheet(tabId, frameId, oldStyleSheet);
 
-  frame.injectedStyleSheets.set(groupName, styleSheet);
+  // The standard style sheet is ~660 KB per frame (as of Adblock Plus 3.3.2).
+  // Keeping it in memory would only really be useful on Firefox, which allows
+  // us to remove it via the tabs.removeCSS API. By choosing not to hold on to
+  // it, we save potentially several megabytes per tab (#6967).
+  if (groupName != "standard")
+    frame.injectedStyleSheets.set(groupName, styleSheet);
   return true;
 }
 
 function getExecutableCode(script)
 {
   let code = executableCode.get(script);
   if (code)
     return code;