Run a full scan of Malwarebytes (free antivirus scanner)
http://www.malwarebytes.org/
If this finds a ton of infections I recommend running combofix(free cleanup tool)
http://www.bleepingcomputer.com/down...virus/combofix

If you have your windows disc on hand, scan your windows for issues with system files.
-open your command prompt
-type sfc /scannow

After you are now sure your system is clean, and windows is not corrupt. Reinstall Firefox, or any other browsers.