May be this will help you...
First you must download Revo Uninstaller..
Visit
http://www.revouninstaller.com/revosetup.exe to download
Then open it and search for Mozilla Firefox in it..
Uninstall Mozilla Firefox Using Revo Uninstaller
Visit
Firefox web browser | Faster, more secure, & customizable and download Mozilla Firefox Newest Version
Restart your computer
Now, open Firefox and see!!!
I hope this will work for you!! (Do the steps correctly!!)