equires: Firefox: 0.7 - 1.6a1
Requires: Mozilla: 1.4 - 1.8
Download and install Adblock Mozilla Extension from.
Code:
http://addons.mozilla.org/extensions...cation=mozilla


After install restart mozilla Click tools at the top menu go down to adblock than click the preferences, enter the ad site you want to block and click add.

EDIT:
Install Adblock Filterset.G Updater too
Code:
http://addons.mozilla.org/extensions...mpg=10&id=1136

This extension automatically downloads the latest version of Filterset.G every 4-7 days. Filterset.G is an excellent set of filters maintained by G for Adblock that blocks most ads on the internet.