Adblock Plus is without doubt the first Firefox addon that I install.
Adblock Plus gets rid of all those ads and banners on the Internet that often take longer to download than everything else on the page!
On installing Adblock Plus for the first time you are prompted to select a filter subscription. The beauty of this is that you don’t have to break you head with configuring which ads to block.
You can always choose to whitelist certain sites. e.g. I’ve whitelisted this blog as well as my personal blog.
You can easily add other ads (which have not been blocked) to block by right-clicking and blocking.