Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of. You can try these free programs to scan for malware, which work with your existing antivirus software: * [http://www.microsoft.com/security/scanner/default.aspx Microsoft Safety Scanner] * [http://www.malwarebytes.org/products/malwarebytes_free/ MalwareBytes' Anti-Malware] * [http://support.kaspersky.com/faq/?qid=208283363 TDSSKiller - AntiRootkit Utility] * [http://www.surfright.nl/en/hitmanpro/ Hitman Pro] * [http://www.eset.com/us/online-scanner/ ESET Online Scanner] [http://windows.microsoft.com/MSE Microsoft Security Essentials] is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one. Further information can be found in the [[Troubleshoot Firefox issues caused by malware]] article. Did this fix your problems? Please report back to us!

Thank you for your suggestion, I've already tried Microsoft Safety Scanner and Microsoft Security Essentials. It didn't turn up anything.

Try start Firefox in Safe Mode: *[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Firefox in safe mode] Maybe your session didnt finish: *[https://support.mozilla.org/en-US/kb/firefox-already-running-not-responding#w_end-firefox-processes End Firefox processes] Try also reset Firefox: *[https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems Reset Firefox]