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Opera was the Xerox PARC of web browsers in a lot of ways.

Except that you could actually use it instead of being a legendary project we only heard about but very few people actually used. That makes it all the more painful that it was taken away.

Yes, I remember Opera having it around that same time, and Firefox having a plugin to copy it. I think the Firefox plugin had full "tab tree" support though. I believe it died after one of the many updates that changed how plugins worked, although I haven't checked in a couple of decades to see if something similar has appeared.

You can still have tab tree in Firefox using Tree Style Tab (https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...)

Same here. I loved OW and its Tab layout. But after its demise I went to Safari.

Vertical tabs are already a thing in Safari with tabgroups although it doesn’t completely remove the horizontal tabs