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You can try Vertical Tabs in Firefox now
(www.ghacks.net)
A community for discussion about Mozilla Firefox.
Wait. Isn't this a Firefox extension that a lot of people like? Not me specifically, but I'm surprised by your comment having a lot of upvotes.
Edit: ah, it's not upvotes, it's downvotes lol!
Yeah, Tree-Style Tabs is the most popular extension, although there's various others, and this has been a highly requested feature for a while.