We're implementing it though: about:keyboard in a Nightly build does what you expect, this is tracked in https://bugzilla.mozilla.org/show_bug.cgi?id=2000731 and dependencies.

No, it doesn't do what I expect, the list of the default rebindable keybinds is small, can't bind multiple shortcuts to a single function, can't bind without modifiers- if I recall correctly after trying it out a while ago.

Thanks for sharing this! Went and changed some keybinds right away.