I haven't seen this particular one, but try getting the adblock extension --
http://adblock.mozdev.org -- it is pretty good at blocking extra-nuisance pop-ups.
Although, FWIW, I think clicking the OK button should be harmless on OS X and Firefox. It would probably even be harmless on Windows and Firefox. God knows what would happen if you're using MSIE though.
🙁
P.S. Spill the beans...whose profile should I not be visiting?
😛