One of my most loved Firefox extensions is AdBlock. It helps keep the web free of ad-clutter and delivers content without being overcrowded by advertising.
Here are two before-and-after examples:
Example 1: moneycontrol.com
One can hardly see any content without scrolling down and hunting for it. moneycontrol.com's news page contains a banner ad, a sidebar ad and a flash banner in the middle.
And here's how moneycontrol.com looks with AdBlock.
Much better, don't you think?
Example 2: rediff.com
rediff.com has really annoying pop-ups which you have to click off first, and then get to the content, which is heavily garnished with banner ads.
AdBlock cleans up rediff.com's article page, making it actually readable, and Firefox blocks the annoying pop-up. Convinced?
AdBlock is available at https://addons.mozilla.org/en-US/firefox/addon/10
I recommend you also install its companion AdBlock Filterset.G Updater, which automatically updates its ad blocking list. The updater is available at https://addons.mozilla.org/en-US/firefox/addon/1136
Here's a nice guide on how (and why) to install Firefox extensions.
Still don't have Firefox? Get it at once from http://getfirefox.com/