Looks like API for webextensions to add keyboard shortcuts is coming to Fx 60.0 - Mozilla bug #1421811!
Tree Style Tab uses it.
Example component used in TST addon.
I had mixed experience in TST as I couldn't assign any hotkey combination I wanted but most of it works.
Keyboard shortcuts are coming to Fx 60.0
Re: Keyboard shortcuts are coming to Fx 60.0
Yep, they're slowly implementing some of the stuff they should've had working months before they did the switch. I'll start using it to re-implement pre-existing Flagfox features in the not too distant future.