Tabbed browsing

Tabbrowser preferences (TBP) is a lightweight extension that provides additional tab-related functionality, including single-window-mode.

Tabbrowser extensions (TBE) is a gigantic extension that changes some core browser files (and is thus not recommended by the Firefox developers, though a lot of people use it). It is very powerful, adds just about everything you could want with tabs (plus a whole lot more), and very likely to break from build to build.