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

lib/contentFiltering.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 143 matching lines @@
   // reappearing momentarily.
   if (styleSheet && !addStyleSheet(tabId, frameId, styleSheet))
     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;
 
   code = compileScript(script, [snippetsLibrarySource]);
@@ skipping 113 matching lines @@
   updateFrameStyles(sender.page.id, sender.frame.id, message.selectors,
                     message.groupName, message.appendOnly);
 });
 
 fetch(browser.extension.getURL("/snippets.js"), {cache: "no-cache"})
 .then(response => response.ok ? response.text() : "")
 .then(text =>
 {
   snippetsLibrarySource = text;
 });