When I close Firefox it seems to take forever and a day to close. It's shows that it's stilll a running process with CTRL/ALT/DEL. Windows XP w/Service Pack 3 and plenty of anti-virus and malware protection.

I close Firefox 4 and after several minutes I'll try to open it again to browse and I receive the message that it's still running?? CTRL/ALT/DEL shows it's a running process still. Windows XP Pro w/ Service Pack 3. Can I revert to 3.6 ??