I'd be fine with letting an extension take over that niche ... if they made an API that let it replace the horizontal tabs instead of only being able to duplicate the tabs in the sidebar. Or even a way to configure that as a user.
That is how tree style tabs worked before Firefox switched to webextensions. And when Firefox switched to the we extensions API, a ticket was immediately opened requesting an API allowing hiding the horizontal tab bar. But despite some initial interest, nothing really ever happened with that.
Yep. While Sidebery, TST, etc have done about as well as they can given the limitations of extensions, the resulting vertical tab experience is half-baked without userChrome mods. It's also still kinda janky even with the mods unless one spends time polishing out all the edge cases, but then there's a high chance the mods will randomly break in weird ways after an update one day. There's real value in having the feature be native.
Hmm but I don't want to use vertical tabs in every window. Only some of them, in the others (with few tabs and narrower 4:3 displays) i want horizontal. If I hide it in userchrome I end up with no tab bar in those cases.
That is how tree style tabs worked before Firefox switched to webextensions. And when Firefox switched to the we extensions API, a ticket was immediately opened requesting an API allowing hiding the horizontal tab bar. But despite some initial interest, nothing really ever happened with that.