The following was taken from a posting on the Mozillazine forums by laszlo.
Posted: Wed 8th Jan 2003 4:23pm How to uninstall an extension
Until the uninstall feature for extensions is implemented (bug 170006), use this procedure as a workaround:
1. Exit Mozilla Firebird
2. Change to the chrome dir of your Mozilla Firebird application dir
3. Delete chrome.rdf and the overlayinfo dir (they are regenerated on next start)
4. Delete <extension>.jar (or the <extension> dir, if it's used unpacked)
5. Remove every line with a reference to <extension> from installed-chrome.txt (usually there are two: one to the content and one to the locale)
6. A few extensions might use additional files; TBE for example has a tabextensions.js in <Phoenix dir>/defaults/pref to store its defaults in.
Search through your app dir for such files and delete them.
7. Restart Mozilla Firebird
It's probably a good idea to make backups of the files/dirs before deleting/changing them to be able to restore the status quo in case anything goes wrong.