Is that the tree-like organization? I have a shit ton of tabs in one of my workspaces, so that's something that would be very useful for me to organize related tabs, tho I should also close a lot of them that I simply have no use for but forgot to close lol.

Considering things like Zen’s default sidebar seemingly being very thin...

I think that by default it now uses full sidebar with compact mode enabled, which makes it hidden, tho I use it without the compact mode. But I don't know CSS to theme stuff, tho there might be themes online I can download.

There is another feature I use a lot on Zen that I forgot to mention, which is Glance, so I'll probably remain using Zen because of that, ~~but I'll test Sidebery on Librewolf later~~ but I'm testing Sidebery on Floorp now, thanks for the recommendation!