Hell despite missing tab groups, Zen browser is the only browser that finally had a "good enough" vertical tabs implementation, which allowed me to finally drop Edge as my main browser.
What do you not like about Firefox's Tree Style Tabs? I might be open to an alternative.
I Cant't hide the tab bar without some userProfile.css hacks, while in Zen it has first class support.
It's more a matter of saving precious vertical space.
I see. Yes, I already hid the stock tab bar years ago and forgot about it.
I'd be curious for your comparison to Waterfox, which added vertical tab integration a while ago.