Download, install and run the free version of Malwarebytes Anti-Malware:
http://www.malwarebytes.org/products/malwarebytes_free/
(NB: When installing, deselect the option to install a trial version of the PRO version).
If that doesn't work, note down all of the extensions you want to keep in Firefox (because you'll need to reinstall them later). Go to Help > Troubleshooting Information and click on 'Reset Firefox'. (Don't worry. Your bookmarks, etc won't be lost).
If that doesn't work, run System Restore to take your computer back to a date before the problem occurred.