I've been using the tree-style-tabs plugin for the last 4 years, because I like vertical tabs, and nesting it provides.
But now that Firefox actually finally has proper vertical tabs, and tab groups, I can move away from tree-style-tabs (I don't use any of its other features).
I like Arc's user experience with vertical tabs. They are bigger, easier to organize and they are cleaner. Also, the sidebar toggle is hard to work with, ideally I would prefer the ability to toggle with a shortcut or reveal on hover.
Aside Arc, Zen browser has a good vertical tab experience.
Overall, I still main firefox for my personal browser, though it's UX is still lacking.
There is a keyboard shortcut. It's CTRL+ALT+Z for me. Unless you mean something else?
As for the "reveal on hover", iirc there was a dismissable message that said it is coming soon.
If I can share my opinion, they are more than big enough if you toggle the checkbox "optimize for touch screen". I would have to try Arc or Zen again to understand what you mean.
The only complaint I have is that I need to hover (or expand) to see the title. It becomes annoying when I'm reading documentation and I end up with multiple tabs with the same icon.
EDIT: I can't seem to find the "optimize for touch screen" checkbox anymore, but I'm sure there is something like that somewhere because I enabled it on one of my devices which has a touch screen.
Go to Settings > Design, and then under Tab Bar Style select "Vertical Tab Bar (experimental)". I recommend checking the "Collapse Vertical Tab Bar" checkbox, too. That way, the tab bar collapses into icons when your cursor isn't over it, taking up less window space.