Rated 5 out of 5 stars
The TinyURL extension hasn't worked for ages (I've really missed it in the last few updates of Firefox). However, there is a working version (Firefox 3.6.8), but it's not on this page!
Get it from here: http://tinyurl.com/2wft2bn
(or http://xsidebar.mozdev.org/modifiedmisc.html#tinyurlcreator )