Download and re-install Firefox.

If you did not select to remove personal data, Tony's solution should work for you.

If you selected to remove personal data, you should try an "undelete" utility such as: http://www.piriform.com/recuva

If you have removed your personal data when uninstalling Firefox then the Firefox profile folder that store your Firefox personal data has been removed. If you do not have a backup of the files in this folder then your only option is to try to recover this data with an undelete utility like Recuva.

• Recuva: http://www.piriform.com/recuva

• https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox