Simplify delayed startup and increase delay a bit
Improve startup performance: load utils.js only if needed
Simplify code to don't handle "popupshowing" from sub-menus
Don't set referer from content.location for text links
Add "useRealRefererForTextLinks" preference
Use readFromClipboard() (if available) and call nsITransferable.init()
(see https://bugzilla.mozilla.org/show_bug.cgi?id=463027, "Implement per-window Private Browsing")