Get AdBlock Plus Rated 3 out of 5 stars
Installed this by mistake - the next day, found AdBlocker Plus which is probably the best one currently (many features, popular - Nov. 2012, Firefox 16.) The add-ons search at addonsmozilla.org failed to find AdBlocker Plus for some reason, when I tried it.
Edited - I'd written that I had spell-checker problems after I installed this, but it had nothing to do with the app, sorry!
P.S. The logic is a fallacy - I am good at reading code but too lazy to find it!
It is not updated anymore. I stopped using firefox so I do not update this anymore. However I do not have problems with the ad-blocker on my laptop that still has firefox. This add-on may interfere with other add-ons.
If you have to ask where to find the source code you would not be able to tell if it is malicious. I can tell you though that it is not. It just scans connections that the website you are on makes and compares it to a list of known ad sites and blocks it if it is an ad. It does not record or send any information.