Tree Style Tabs is used by like half the folks working at Mozilla.

I doubt that's the reason.

For some reason I really doubt that statement.

I work at Mozilla.

The other half use tab center?

shrug lots of folks don't use tab addons. But yes, Tab Center (Redux) is significantly used too.

maybe a bit of dogfooding and getting everyone to try tree style tabs would help make Firefox a better browser.

Then care to explain why we still can't hide the top bar as a setting?

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.

They work on it. The main problem is simply that Mozilla fucked up the transition from legacy addons to the new system. There are so many APIs and functionallity still missing, that most commercial companys would have been killed over it. But with OpenSource this has become kind of normal to accept it. Just Remember KDE 3 -> 4. Python 2 -> 3. Gnome 1 -> 2 -> 3. Breaking things is just to common now.