If your not having any joy when installing addons in Firefox 3 (sorry can’t remember the specific error message) – but nothing seem to fix it – even re-installing) we managed to confirm the fix today at work. Note that involves completely clearing all Firefox related info (so settings, bookmarks etc will be gone)…
* Uninstall Firefox
* Then make sure that the C:\Program Files\Mozilla Firefox is completely gone
* Also clear out the C:\Documents and Settings\<<username>>\Application Data\Mozilla folder (it will probably still contain Firefox and Extensions directories)
Voila – you can install extensions again (like the brilliant FireBug)
This also works for Linux (had a similar issue at home with the Latest version of Ubuntu).