I've just been looking on the net and it appears to be an incompatability with old personal info and FF3. someone has found a solution.

the only way it working out is to remove (rename) personal info in Firefox (documents and settings) and do a complet reinstall. (you can use the Firefox addons (or even Mozilla backup tool) for your bookmarks & passw file to reinstall do this before you remove your old Firefox version, you need those files for import again.
Also if you have the errorzilla addon that can do a similar thing.