Remove firefox under ad and remove programs also under programs delete any residual files from firefox including cookies and internet history etc. restart you pc and reinstall Firefox.

Try updating your Firefox to the latest version. Here is where you can download it:

http://www.mozilla.com/en-US/firefox/upgrade.html

If it does not help, then you may need to reinstall Mozilla Firefox from your computer by removing the existing one first before installing the new version again.