KeySnail (free) adds a wide array of features to the Firefox Web browser that are accessed using a sequence of keystrokes, such as Ctrl-x followed by b, rather than one hotkey.
The keystrokes are based on Emacs, a somewhat complicated but powerful text editor for Unix-based systems such as Linux. Some commands allow for rapidly skipping around a Web page or among open tabs, while others offer mode-sensitive editing features that work only while in a text entry area, for example.
While some useful commands won't require any pre-requisite Emacs knowledge, such as hitting F3 to record typed text and then F4 to type it all back when needed, KeySnail is largely aimed at those who already know Emacs well. Trying to figure out the add-on without already knowing how to use the Unix editor would likely prove frustrating, to say the least.
But if you already know and love Emacs, you might find plenty to like in KeySnail. You can define your own key bindings to supplement its large number of built-in commands, or install a plugin to take it even further. One plugin makes KeySnail behave like a twitter client, for instance.
If you give it a whirl, keep in mind that KeySnail's key bindings will override defined hotkeys for Firefox or a given Web application, such as Google Docs. You can quickly suspend KeySnail by hitting F2 to use the original hotkeys, and bring it back with the same key.
If you instead wave the vi banner in the never-ending editor battle, take a look at Vimperator. It wraps a vim-like control scheme and interface around Firefox.