[V-2]

Ah, ok. They are however available in Firefox as an extension, aren't they? Or at least that used to be the case

[arupauqa]

I tried the Vivaldi version of this a few months ago and it unfortunately does not meet the feature set of Tree Style Tabs. It isn't a worthwhile alternative, for me.

[FreeFull]

Yes, they are available with the Tree Style Tabs extension