Firefox invented vertical tabs but then forgot about them. Always having to search for the next short lived plugin made me switch to Edge. I'd be delighted if this would change
FWIW "Tree Style Tabs" has been around a long time, even making the jump to web extensions early.
It's not perfect, but the level of UX polish and the first-class integration with other browser features is unmatched by any Firefox vertical tabs extension I've tried.
I still can't believe the other browsers haven't caught on to the fact that power users want vertical tabs and are willing to switch browsers for it after so many years of Firefox extensions showing the way...
I used OmniWeb.
Currently Firefox is the only browser that I can get an addon to provide vertical tabs. IUt is called Tree Style Tab