Art, I had a virus that did that one about a year ago, I uninstalled Firefox and reinstalled but even that did not work. I believe it came from a free program that I downloaded that forces you to install other crapware. even after un-install of the free program and it's crapware Firefox was playing dirty. I had to do a System Restore just to fix Firefox.
Speaking of Firefox, My boys gave mine a virus yesterday on my laptop, while playing games.
Symptoms
1) every time I goto a new website window avast stopped a hazardous action.
2) every time I goto a new website anywhere from 2 - 20 new windows open.
note: Symptoms only happens with Firefox, not IE or Opera.
Edit: had to fix with Firefox re-install, all is well now
