Using windows 7, Firefox version 6.0.
Start Firefox in Troubleshoot Firefox issues using Safe Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).
Do a malware check with some malware scanning programs.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of their databases before doing a scan.