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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
That's coming: https://connect.mozilla.org/t5/discussions/firefox-sidebar-and-vertical-tabs-try-them-out-in-nightly/m-p/64263/highlight/true#M22608