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

For example, I'd love to be able to press ctrl-a to get to the beginning of a line in an input field and ctrl-k to delete that line, etc.

P.S. Not sure if it matters, but I use Arch Linux.

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.

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.

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

Firefox normally uses the GTK settings for key bindings. You can just turn on Emacs bindings there GTK-wide. See http://kb.mozillazine.org/Emacs_Keybindings_%28Firefox%29#GNU.2FLinux