How do I map a keyboard shortcut with *more than two modifiers* to a FireFox extension?

Question

I have a "hyper key" on my keyboard; which holds all of the modifiers (⌘⌥⌃⇧) at once; it's useful to map 'global' hotkeys that won't conflict with any (reasonable) application defaults or existing muscle memory.

Unfortunately, Firefox likes to complain when you try and use more than two modifiers:

The only note about this I could find anywhere regarding this was an oblique reference in the documentation for extension authors:

Key combinations must consist of 2 or 3 keys: 1. modifier (mandatory, except for function keys); 2. secondary modifier (optional); and 3. key (mandatory)

Is there any way to circumvent this restriction? Why does it exist? Are there any other resources on the topic that I've missed in my research?

Answer 1

This is a bit old, but as its one of the only results that comes up for this, I'll add my (not perfect but usable) workaround:

Leave the default extensions shortcuts, or if those conflict with something else for some reason, change them to any two modifier + one key shortcut that won't conflict.

Then using Better Touch Tool (which is where I'm using my hyper key anyway), add Firefox specific triggers using your preferred hyperkey shortcut as the trigger, and the extension shortcut as the action.

For example if you wanted to use HYP + D to toggle Dark Mode, your BTT trigger would look something like:

⌘⌥⌃⇧ + D --> ⌥⌃ + D

I'm sure there are some downsides to this method, but it seems to mostly work!