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Tree Style Tabs should continue to work. It's a very popular extension. Mozilla should probably include a disclaimer about Tree Style Tabs still working with any blog post about WebExtensions. :)

Mozilla is now beta testing a prototype of native Firefox vertical tabs, though it is not as advanced as Tree Style Tabs:

https://testpilot.firefox.com/experiments/tab-center

Vivaldi has a nice Tree tab style that I constantly use, it has become second nature - we're even looking at replicating it for our enrollment software, crazy enough.

they have a bug that tracks this use case

https://bugzilla.mozilla.org/show_bug.cgi?id=1208596

and a larger in scope bug https://bugzilla.mozilla.org/show_bug.cgi?id=1315819

Chrome used to natively support tree-style tabs. Unfortunately they were removed a few years ago.