Have a look here:
Basic Troubleshooting | Troubleshooting | Firefox Help and here:
Uninstalling Firefox | How to | Firefox Help
Just using the standard uninstall procedure doesn't remove your profile, which
could be the problem. Note I said could - it may be one of the other bugs mentioned in the first link, so give them a try first.
Many programmes leave all sorts of files/folders/registry entries behind, I use RevoUninstaller if I want to completely rid my PC of something...