From the archive: originally published on blog.cronky.net in 2008. Some links may have rotted.

Fix for Firefox Addons error...

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) * Re-install 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).