I only now started getting this problem. It didn't happen until this morning, when I turned on my laptop, and Safari became really slow and unresponsive. I don't know how many times I had to force quit the bloody thing. And to my great frustration, there were ads out the wazoo by MacVx. I'm glad this topic was up, or I would've driven myself insane eventually. Problem is I don't remember installing a plugin with this particular name, or anything similar. But there it was, clear as day in Preferences. Keep an eye out for MacVeXe, which is the name that infernal plugin had in my case. Once I got rid of it, the ads disappeared, and things are running much more smoothly now. In Firefox, it's MacVx. Oh yeah, I almost forgot to add that I tried Firefox earlier, and had a similar problem. Got rid of that bloody thing, so if I use Firefox again, I hope I won't have to deal with this mess again. I don't know how many other names the infernal thing may have, but those were the ones that I caught, after reading this thread.
EDIT: I guess the auto censor bleeps even mild invectives. Eh, no matter.