The only thing those Firefox extension lack is the ability to hide native tabs (I understand that this is a limitation of Firefox extension APIs). You have to manually modify your userChrome.css to do so.

I swear they said they were going to add hiding the top tab bar back in as a native option back in 2017, but I'm still using the userChrome.css hack today.

There's a long bugzilla thread since 2017 exactly for this. It has a some activity over the years, but was never prioritized. https://bugzilla.mozilla.org/show_bug.cgi?id=1332447

You mean hiding the top bar, right?

Yes, that's what they're talking about. It ends up wasting space to have your vertical tab sidebar without the ability to hide the default tab bar at the top of the window.