What does the little popup blocker look like? Firefox usually displays an infobar below the address/location bar with an Options button. If you are seeing something else, perhaps it is a feature of another add-on? I would especially check any toolbars installed by your antivirus or other security software.

It's the little red circle with a line through it. I think it's the Adbllocker one but I deactivated that. I haven't seen the infobar for anything since I downloaded ff4.

Thanks for the quick reply.