How much could be adapted through userchrome.js mod?

[BoomerangAide]

Hi,

I'm using https://github.com/Aris-t2/CustomJSforFx that make use of an userchrome.js mod to apply changes that cannot be done through web extensions.
I was wondering how much of this project here could be adapted, if it is possible, to work using this feature that provide access to browser stuff web extensions don't have, and that cause them to be inferior to pre-Quantum tools.
It may not offer menus, and require options to be hardcoded by the user, but maybe many things could be restored?

[BoomerangAide]

Someone in Aris-t2/CustomCSSforFx#427 (comment) mentioned something that seemed like a project to revive old legacy extensions.
Since I failed to see exactly how that work, I thought maybe only an extension author (@simonlindholm) may understand if that's relevant to his work, and do something in this case, so I'm mentioning it here.