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Firefox Sidebar and Vertical tabs: try them out in Nightly Firefox Labs 131 – Firefox Nightly News
(blog.nightly.mozilla.org)
This is a most excellent place for technology news and articles.
Years ago, I used an extension called Tree Style Tab. It's better than just vertically displaying tabs. New tabs open as children, and you can collapse branches, so things stay organized when you have a ton of tabs.
Another popular one is sideberry, which is a variation of the same thing. :)
I use sideberry in floorp which has been great!