Can you delete them using explorer?
I had this issue. I could not delete using Firefox 2.0.0.12 nor FF 3 Beta 3 on my XP box, but Internet Explorer on the same machine worked OK. Also FF 2.0.0.12 on my Ubuntu box worked fine.
The fix was to uninstall firefox (both versions) on the XP machine, delete the stray folders (in progam files and application data) and reinstall it. Works like a charm again, but replacing the extensions again is a PITA
HTH
Blake