Is there a Firefox add-on that enables emacs keyboard shortcuts?

KeySnail also looks extremely interesting.

(In fact, after a cursory look at both that and Firemacs, I would say that KeySnail looks like the more comprehensive of the two by far.)

Other useful links:

• It's All Text extension for Firefox.
• Mozilla page at the EmacsWiki.
• Edit with Emacs extension for Chrome.
• EmacsClient page at the EmacsWiki, for other client tricks.
• Conkeror: A Mozilla-based web browser with Emacs-like functionality.
• Use Emacs keybindings everywhere.

Yes, this should be what you want: https://addons.mozilla.org/en-US/firefox/addon/firemacs/ and of course you can customize the bindings if need be.

Not a plugin, but you may get some mileage out of conkeror.