If you thought Firefox was great because it allows users to “take back the web” from poorly written, intrusive, and annoying web sites, you ain’t seen nothing yet! Greasemonkey allows users to create scripts that modify web pages on-the-fly as they are loaded in your browser!
What does that mean? It means you can remove all articles about Michael Jackson from Reuters web pages. It means you can make all links to NY Times articles load the print version of the page. It means you can strip ads from Google. And much, much more! The whole list of currently available scripts is here. Some of them make pages easier to read, others add functionality, but be careful before installing these scripts, since there is certainly some potential for mischief in such a powerful tool. Still, I don’t think the web will ever be the same!
If you only wish to block ads, I recommend AdBlock, a more focused Firefox extension. It allows you to control-click on any image, iframe, flash movie, etc. and then make sure it is never loaded!
Here are some other useful Firefox Extensions I’ve found. Although I am not a big fan of Firefox’s non-mac-like GUI, all of these powerful extensions put it in a class by itself among web browsers!