Thanks for your rating! I have good news, it is already possible to customize the hotkey! By default this functionality is a bit hidden in Firefox, unfortunately.

Here's the process:
1. Navigate to about:addons
2. Click the settings gear ⚙ at the right-top
3. Select "Manage Extension Shortcuts"
4. Change the shortcut for Fullscreen

We'll try to make this more accessible & provide better documentation when we find time. Please check out this GitHub issue for full context, and a way to do the same in Chrome:
https://github.com/MarkTiedemann/fullscreen/issues/2#issuecomment-583909873