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You asked, we built it: Firefox tab groups are here | The Mozilla Blog
(blog.mozilla.org)
A place to discuss the news and latest developments on the open-source browser Firefox
I gave a few of my personal use cases above, but in short: when I need to reference or act on multiple things on different sites at short notice, and will probably need to again later; to label tabs; and when I need multiple tabs of the same website, but because the URL doesn't update a bookmark is insufficient.
Edit: You're welcome!