Your PC is infected with malware and highly dangerous spyware. Download Avast! Free Antivirus from here (http://download.cnet.com/Avast-Free-Antivirus/3000-2239_4-10019223.html?part=dl-85737&subj=dl&tag=button) and install it. Don't register if it asks to, you have 30 days left for registration. Go to SCAN COMPUTER > Boot Time Scan and click on the big "Schedule" button. Restart your PC. Avast! Boot Time Scan will initialize detecting viruses in real-time. Delete any found viruses, malware, spyware. Once the scan is over you can try Firefox again.