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Firefox Sidebar and Vertical tabs: try them out in Nightly Firefox Labs 131
(blog.nightly.mozilla.org)
A place to discuss the news and latest developments on the open-source browser Firefox
I've gotten so used to Sideberry that I'm not likely to switch to the built-in vertical tabs once it gets released.
My main issue with a non-native solution is that you need use userchrome hacks to make the horizontal tabs bar disappear, which doesn't sync between installations. I use Firefox in multiple computers at the school I work at, so it isn't a viable solution.