Welcome to the forum.
Did you download firefox from its home page?
Did you restart after uninstalling it? (Not necessary) But in some cases it works.
Download Ver 14.0.1 from the link below, but Reboot
your computer before installing. Download Firefox 14.0.1 - FileHippo.com
Also do you have Ccleaner installed?
Go to the Registry
tab | Scan for issues
| Fix selected issues
| You will be prompted for keeping a back up, hit yes if you do, if not hit no and Fix all selected issues
. Reboot then try install firefox.
Do post back results.