If you close Firefox, then start Firefox in Safe Mode, does the problem still occur? For instructions on starting in Safe Mode, see Diagnose Firefox issues using Troubleshoot Mode.

Yes, I followed instructions in safe mode.Still the same.

If you use the Finder to go to your Applications folder, is Firefox there?

Yes, it's there. I am using Safari for the moment. It's strange cos this is on my laptop. When I put Snow Leopard on my IMac I lost Firefox then.

Are you running a firewall or anything that might be getting in the way?

No.Firewall is turned off.

Try reinstalling Firefox. How to download and install Firefox on Mac

Since updating to Snow leopard I cannot download.When I click on download it changes to a grey sheet. I can't do a software update either as it tells me am not connected to the net when I am. Could this all be part of the same problem?

Yeah, it's probably the same issue. When I updated to Snow Leopard (from Leopard) I didn't have any problems. Maybe the folks in the Apple forums might be able to help. http://discussions.info.apple.com/category.jspa?categoryID=263