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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
Sidebery is fantastic, but I do feel like it's making things a bit slow. I might be prepared to give up on panels and nesting if I can get better performance. (And possibly the Tab Groups work can replace my use of panels.)