I use Chris Pederick’s Web Developer toolbar (FireFox addon) all the time, it’s a must for any designer/developer looking to debug HTML/CSS. However it suddenly disappeared after I updated Firefox to 3.0.6 and Skype to version 220.127.116.11.
After frantically hunting for a solution I finally found out what the problem was. Skype naughtily installs a FireFox addon which appears to be uncompatible with the Web Developer toolbar for some bizarre reason.
Simply going to Tools » Addons in FireFox and disable or uninstall the Skype extension for Firefox 18.104.22.168 and viola, your toolbar is back in place!