aah, I have a popup window ad - how to go about getting rid of this

for 2 years, I've used my mac with firefox and really NO popups.

Now I have a popup that keeps coming up.

How do I go about detecting which website is bring it up?

I don't usually notice it until I cycle through my windows because sneakily - it doesn't Pop to front of screen, it pops behind my webpage... sneaky buggers.

my hunch right now is it is like imageshack or a website like that. I've been looking at posted screen shots recently and think it is something like that.

Thanks for your tips!