FWIW, there's at least one (minor) change that was made to Firefox to help out Tridactyl - https://github.com/tridactyl/tridactyl/issues/792.

Mozilla also were happy in principle to allow us to intercept key presses on all parts of the browser last time we spoke to them a couple of years ago; we just need someone to write that extension to the WebExtension API - https://github.com/tridactyl/keyboard-api.

Rewriting Tridactyl in XUL is not something I would wish on anyone.