Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on...
The Web Developer extension adds various web developer tools to the browser.
View HTTP headers of a page and while browsing.
An extension that allows for the use of dynamic commands in Firefox.
Comfortable surfing with a keyboard. This extension provides some functions that you can use to make your browsing with a keyboard more comfortable. Next/prev page (e.g. in Google search results), smooth scrolling, etc.