I seeked into the API in order to write a plugin for myself. In order to aid my workflow I need to listen to two keystrokes closely followed.
Apparently there are only hacks available.
I want to fire-and-forget about my plugin as long as the API to command firefox stays consistent.
I did not attempt to write any code (and any JS code, honestly that is) yet. Simply because I did not get that requirement addressed by the official API /& documentation.
Recompiling firefox would be the least favorable option; But I would still be hooked if it would enable me to observe keystrokes on my own.
As an practical example:
Pressing Shift
two times in sequence and within 100 ms should enable me to eat the event.
Probably you can inject your js and add an input event in the body of the page.