Firefox blocks popups by default, the adblock extension does not do this.

Adblock is for specifically blocking images, i-frames or .swf's

You have to block each individual image/ad with adblock, it doesn't block them all by default.

I use it to block almost all extraneous advertising imagery from most websites. This greatly reduces page load times and bandwidth used.

The way to circumvent this Robson, is to use css's background image tags.