As usual, Mozilla doesn't come up with new things. Only when Floorp and Waterfox start doing stuff do they think "Oh wait, people actually like that? Well, let's kill an extension or fork!".
Tree Style Tabs have existed for years. I guess it took them a while to wake up.
Christ. If they don't do vertical tabs people act like it's the end of the world and Firefox is a piece of shit browser that deserves to die, and when they do they aren't being original enough!
They can't win. What do you actually want? Diagonal tabs??
The very first version of Tree Style Tabs was published in... hmm...
2007
The shameful part is the fact that Edge-Chromium added a native tree style tabs feature over three years ago, and has been eating Firefox's lunch. Vivaldi has had native vertical tabs for eight years! Mozilla's leadership is asleep at the wheel.