While I am not entirely sure of what you are talking about, it sounds like you are trying to define shortcuts or overrides in X11/Xorg input settings for your input device. My guess is, based on the age of the Firefox port on *nix, and the fact that X is basically dead upstream - is that even if this is a valid bug, you probably won't get a fix very quickly.
Why? It isn't Wayland and I'm guessing the abstractions here are cleaner.