gtk-key-theme-name = "Emacs"
This will give you many Emacs key bindings, specifically C-a, C-e, C-f, C-b, C-k, C-u, C-w, C-h, and C-d. However, you won’t get C-y, C-p, C-n, C-s, C-r, C-j, C-m, and C-l, since they’re overridden by menu item accelerators.
To get those key bindings too, go to about:config and set ui.key.accelKey to 18. This switches Firefox’s accelerator key from Ctrl to Alt. So, you’ll need to use Alt-L to go to the URL box, Alt-T to open a new tab, etc…but you’ll be able to use all the Emacs keybindings burned into your muscle memory!
Thanks to David Chandler for this tip.