The way tree style tabs worked after they broke it was never very good. Floorp is what to use if you wanted side tabs on Firefox.
That said I still went back to Vivaldi after trying to use Floorp because of stupid little ux issues like pinned tabs not being protected from closing, and broken session saving.
The issue is that because they broke the UI customization that allowed for it all the extensions are just a kludge to add a panel to the side without actually getting rid of the top tabs.
I remember back in the day (FF 4?) I had the window buttons, tabs, back/forward, URL bar, etc all on one row, which was pretty cool. So it was something like this, from left to right:
Firefox menu - was Firefox, but now would be the hamberger menu
back/forward buttons
extension butons
URL bar
tabs
It worked pretty well. It would be nice to do that again.
I have been running vertical tabs for a while now and it's broken about 3 times, once every few months. Currently, I've had no min/maximise/close buttons for about a week because I can't be bothered to fix it. Far from "one and done".
That's unnecessarily dismissive. Unfortunately, even the best extensions have their downsides. Some used a browser that suited their preferences better instead, which is a shame for both Firefox and the user, in my opinion.
Mozilla recognizes this and is finally taking action to integrate highly requested features into Firefox. Many "who really care" are glad for this, because it is a good thing.
I am, specially after seeing how well it was implemented in the nightly version. It can't be compared to an extension that enables the same capability.