One of the best known and simplest Firefox add-ons, AdBlock Plus effectively blocks online ads from a predefined list of ad providers. You can quickly personalize that list by clicking on the AdBlock Plus "stop sign" button it installs on either your navigation toolbar or your status bar. Adblock Plus runs unobtrusively in the background, and right-clicking an ad brings up message box, through which you can easily add to the list of blocked ads. Removing an ad from the blocked list also is as simple as clicking and choosing the appropriate option. Via the same list, you can customize certain sites and pages to show their ads, but keep all others blocked. This is especially useful for when AdBlock Plus interferes with streaming videos that air short ads when you play them.
While similar extensions exist for Firefox, none has the reputation of this one, which will also work in Thunderbird and SeaMonkey.