I echo your general sentiment about Zen. Zen's default sidetab is pretty bad (no hierarchical folders in 2025 is wtf), and it doesn't play nice with Tree Style Tab to the point where I have to disable it.

I do recommend you take a look at Sidebery. Echoing your sentiment, I can't go back to other tab UI, including that of Zen (especially from before the tab folders) voluntarily.

What is the difference between Sidebery and Tree Style Tab? Or for that matter, how does Floorp compare with the other forks like Zen and Librewolf?

[–] Tomorrow_Farewell@hexbear.net 2 points 1 month ago* (last edited 1 month ago)

What is the difference between Sidebery and Tree Style Tab?

I initially went from using TST (which is suggested in the settings of Floorp 11, and there is some supposed support for it there, though, probably not in 11 anymore) to Sidebery because TST was frequently 'forgetting' the tree structure upon reloading my browser. The TST issue is probably caused by a large amount of tabs open.
Sidebery also supports workspaces ('panels') and tab folders (basically, tabs that - when you view them - show their children in a way similar to how you would view items in a file folder).
Do not remember if TST supports snapshots.
IIRC, Sidebery also has much better in-built customisability (as well as css configurations that are already there, such as for making tab titles multi-line).

Or for that matter, how does Floorp compare with the other forks like Zen and Librewolf?

Floorp is largely a basic fork of Firefox, with the only unique feature that I do not know how to reproduce in other ("standard") forks being the second sidebar that allows for using websites and some other things in a split-screen configuration. The downside is that that sidebar makes complex Firefox css themes (such as ShyFox) not compatible with Floorp without fiddling.
Also, going to note that I am not familiar with the current state of Floorp 12, but I found it unusable for me when I did try it, so I am using Floorp 11. If one doesn't care about this split-screen feature, I'd argue that one would be better served by Librewolf or another fork.

I have tried Zen, but I could not make it work for me. Its sidebar lacks in basic configurability such as width, lacks in features of Sidebery (and Sidebery doesn't seem to work in Zen as of currently). I have not found any features of Zen that aren't reproducible in other forks (Glance was mentioned, but that, too, seems to be reproducible via existing Firefox extensions in other forks), so all it seems to have is beautiful UI, but that's where css themes come in play in other forks.