Would have rated 5 stars but it no longer works in Firefox 22.
Even after manually disabling ALL other extensions, the same symptoms as everyone else: see "## cookies deleted" but does not close tab. Also does not work in Firefox (Beta) 22b6 (have not yet updated to 23b1), Firefox (Aurora) 23a2 (have not yet updated to 24a2), Firefox Portable (PortableApps) 22, all with separate profiles.
Maybe something to do with New Private Window and changes made by developers to accomodate that feature??
Cleared Error Console, click button, in Error Console I get this
Timestamp: 6/28/2013 05:24:42 PMError: TypeError: Components.classes['@mozilla.org/browser/global-history;2'] is undefinedSource File: chrome://closeforget/content/closeforgetcodeOverlay.jsLine: 71