Malwarebytes' Anti-Malware Free
   SuperAntispyware Portable Scanner
   Microsoft Safety Scanner
   Anti-Rootkit Utility - TDSSKiller
   AdwCleaner

Maybe it's a virus and you can see here: [[Troubleshoot Firefox issues caused by malware]] If your anti-virus and anti-spyware software hasn't detected malware, scan your system with the free malware scanning programs listed below. You should scan with all programs because each program detects different malware and make sure that you update each program to get the latest version of their databases before doing a scan. Warning: Anti-virus and anti-spyware software may sometimes generate false positives. Consider quarantining suspicious files rather than deleting them. Note: Certain malware components can only be removed in Windows Safe Mode. Malwarebytes' Anti-Malware Free SuperAntispyware Portable Scanner Microsoft Safety Scanner Anti-Rootkit Utility - TDSSKiller AdwCleaner