Hello. Try reading this support article: https://support.mozilla.com/en-US/kb/Firefox+will+not+start I hope it helps.

If you are still having problems then it is possible that the virus hasn't been removed complete or that the removal has damaged your Firefox installation. It is always better to do a clean reinstall after a virus infection if that involved deleting files by other programs or the use of System Restore. See http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall --- Do a malware check with a few malware scan programs. You need to use all programs because each detects different malware. Make sure that you update each program to get the latest version of the database. http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware http://www.superantispyware.com/ - SuperAntispyware http://www.safer-networking.org/en/index.html - Spybot Search & Destroy http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page .......... See also "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked