https://addons.mozilla.org/en-US/firefox/addon/4364
This is add-on ads the ability to create element hiding rules in adblock. Since many sites and companies use text ads and inline pages instead of flash and plain images this makes a big difference.
For example I was able to remove the HUGE header ad on hotmail.com along with the space it leaves behind. Often removed ads would leave big blank spaces on sites, but if you also remove the container element it really cleans things up.