It's happening (as a WE API) but it's taking a while because they're doing it properly, and a lot of the browser expects the tab bar to be visible.

Folks are working on it, I've been part of a couple of discussions about this.

https://bugzilla.mozilla.org/show_bug.cgi?id=1332447

This comment kinda sums up why it's a hard problem https://bugzilla.mozilla.org/show_bug.cgi?id=1332447#c116

There's a major problem with addons doing stuff the users don't want, and the tab strip is a critical portion of the user interface so "just let them hide it" has problems with abuse. So it needs to be something that can still be autoreviewed, and it needs to be something that gives clear indications to the user as to what is going on.