Perhaps the number one reason Firefox is so popular is due to the Adblock extension. Adblock filters out advertisements from websites. To download Adblock, go to the Adblock page located in the extensions portion of the Firefox website. Once you install Adblock, you'll also want to install the Adblock filterset.G Updater which is an extension that updates the advertisements to block automatically!
Nothing else is required to configure this extension, although you can play with the options by going to tools -> Adblock. Next I will highlight themes and my favorite one, which is also the most popular.