This should come standard with Firefox Rated 4 out of 5 stars
Adblock Plus is THE essental weapon in taking back the Net from the advertising scourge. Anyone who has tried to read a news or special interest site while fighting the distraction of all the infuriating jiggling, flashing, animating ads trying to worm their way into their subconscious will find themselves in Internet heaven the moment they install this extension.
Every time someone asks me to fix their computer (usually defragging, de-virusing or re-installing) I always replace MSIE with Firefox by default - and immediately put Adblock Plus on as well. I generally do this without telling them, and those that noticed the change afterwards have all, without exception, thanked me for doing so, and said that their Web experience has dramatically improved.
The only tiny niggle I have with Adblock Plus (or more accurately, its filter set) is that text-only Google Ads are blocked by default as well. I can understand this in light of privacy concerns, but I also accept that advertising is essential for some sites, as it's their only form of revenue. Text-only Google Ads is a responsible form of advertising that is non-distracting and non-intrusive, and webmasters should be encouraged to adopt this method of advertising. It is the animated, content-obscuring and intrusive Flash/Java/gif ads that need to be blocked, to send the message to the advertising industry that aggressive, in-your-face advertising is neither acceptable or socially responsible.
This said, Adblock Plus should be included as a default plugin with the Firefox install (with a "don't install" option if you don't want it, although why anyone wouldn't want Adblock Plus is beyond me!) It makes surfing the net user-friendly and enjoyable again. It is, beyond doubt, the single best and most important plugin for Firefox there is.