Any Firefox-based browser can use "Tree style tabs" it's vertical tabs from the time before they were cool. Very customizable.

If you right click on the tab bar on regular Firefox you can enable vertical tabs. I don't think they're as nice as Zen's vertical tabs but they're still pretty good

I don't use or care that much about vertical tabs, and it seems complicated how big if a deal this actually is, but florp might be worth taking a look at if you're not already familiar with it

I don't know a ton about it but I think it has a similar kind of niche and is more vertical tab focused

Floorp has literally been right there the whole time.