Rated 5 out of 5 stars

Just checked it under Firefox 3.5.3 - it won't add it as it says it's incompatible, but when you use the download link it will happily add it and it works perfect. Thanks to Daduing for the link... :)
http://ftp.mozest.com/addons/extensions/boss_key/boss_key-0.2.0-fx+fl+ns+tb-windows.xpi

This review is for a previous version of the add-on (0.2.1).