I like tree-style tabs, and I use it when working on projects, but I find that it is a long, long way from what it could be, which is probably a direct result of the lack of competition

the settings UI is absolutely awful;

depending on which appearance you choose, it either looks ugly or blurs together;

it frequently has visual glitches;

could certainly be more customisable appearance and shortcut-wise;

doesn’t allow you to have a title or marker as the top level tab;

as far as I can tell doesn’t let you save a session of tabs;

has an unpleasant glitch where if you drop a tab in the wrong place it’ll disappear behind the current window and never be reachable;

the new tab behaviour irritates me;

and probably a few more complaints that I’d have to actually open it up to remember

Sideberry is a worthy competitor : https://addons.mozilla.org/en-US/firefox/addon/sidebery/

Sideberry lets you turn off the "tree" feature and just have the vertical tabs.

The whole point of vertical tabs is so you have enough characters visible to tell what the tab is. Tab trees waste this precious space.

Sideberry is more complete out of the box. TST is more flexible for low level tinkering. That's what I read somewhere for TST, I use Sideberry.