Easily the most versatile and customizable privacy tool available for Firefox. And since it is developed independently of web-based advertising companies, it is by far the most effective at giving you control over who collects your data while you browse. It can work with or without intense intervention: you can let uBlock make your content decisions for you or you can decide exactly what connections your browser makes for every site you visit.

That said, this is a geek's paradise of a tool. Although it is possible to use it--and use it effectively--without hand-editing the cryptic rules lists, for the highest levels of privacy one has to at least understand the model it uses for deciding whether to block or unblock connections. But the English documentation is very clear, so if you are up for a short learning curve, uBlock can give you very fine-grained control over what you see and over what sees you.