Usually "Flame" does this (a virus that should have been auto destructed 2 weeks ago). It could be another virus or worm. My advice, FIRST scan your computer with antivirus (completely). Don't use free antiviruses. They are not providing enough protection. Then delete all temp. files and browsing history. Remove and reinstall Firefox. (don't install it automatically: ask for custom install at the beginning). It is much better to download it again from www.mozilla.org because your actual version might be corrupted by the virus (IF ANY VIRUS IS FOUND). Anyway you won't lose downloading it again. PS: Windows 7 includes a very useful tool to scan your PC. Windows Defender. Run it along with the antivirus.