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Firefox 124.0 Release Notes
(www.mozilla.org)
A place to discuss the news and latest developments on the open-source browser Firefox
For others who were confused, it's the icon to the far left of the tab bar.
I've used it on my Android phone (open list of tabs, it's at the top next to the private tab selector), but never realized it was also available on desktop Firefox.
Neat!
I actually hid it after thinking it wasn't very good, but that was several versions ago (with a different icon). Thanks for pointing that out, and I updated my comment!
https://support.mozilla.org/en-US/kb/how-set-tab-pickup-firefox-view