A window will pop up periodically on my screen saying:
"Safari can't verify the identity of the website:
's-static.ak.facebook.com...... Show certificate?....Continue?".

It happened even though I didn't have a Facebook tab open and had done nothing to trigger such a message. Of course I didn't go there (at least not on purpose).

Several days ago it was my Firefox browser that was showing this pop-up. During the same period, some of my Firefox bookmarked links to other sites started asking for certificate verification, but only on Firefox on my MacBookPro (OS X 10.7.3), not on Safari, and not at all on my non-intel G-4 Powerbook (OS X 10.5.8)

Also during this period I was getting MacbookPro screen freezes from which the only exit was to power down the computer. Deleting and reinstalling Firefox app didn't help. It was only when I trashed my Firefox profile that the problems stopped.

I ran a complete disc scans with Norton anti-virus and with Sophos anti-virus; also ran the script checking for Flashback. Came up clean.

Things have been quiet for the last few days. But now that pop-up is appearing on Safari; fortunately, no screen freezes - yet.

What's going on? What should I do?