I was using the Firefox browser on my Apple computer and had just used Google to look up someone on the web, and suddenly a message appeared that I had a serious security problem and started to do a scan of my system, even though I didn't request it and didn't click on anything. I couldn't close the web page, and couldn't close Firefox, and couldn't even shut down the computer on the Apple menu. I had to force the computer to shut down using the start key. Every time I restart and try to use Firefox I get the same problem. I'm now on Safari. Help!