A pop up blocker recommended for firefox is 'Adblock Plus'
It is only compatible with Fire Fox 3.0, if you dont have the 3.0 version, downlaod it from this site Fire Fox Home Page
The pop up blocker can be downloaded from this site below.Pop Up Blocker - Adblock Plus
Just click the 'Add To Fire Fox' button, wait 3 seconds and click install. Restart Fire Fox and youre done.
Hope that helps