piroor/multipletab — pull request #41 by Infocatcher (2013-04-13 17:49:15+00:00)
Title: Support for per-window (and per-tab) private browsing mode

See http://dxr.mozilla.org/mozilla-central/widget/nsIClipboardHelper.idl#l40 (I can't find any other documentation).
Or see chrome://browser/content/nsContextMenu.js