"Request for Update" freezes computer

When browsing using Safari, I got a message:

http://www.footimat.info
Warning: Please install updates to continue
[box with OK]

This freezes the computer and I have to restart, with shift key down, to get rid of it. This has happened several times.

How can I prevent this?

You are being tricked into installing malware. Shut down Your browser, reset it, download the free malware bytes adware fighter, and use that to clean your browser of any adware or malware. Don't visit the site where you picked this up again.