I used Tree Style Tab but I didn't like having to change the userChrome.css to make it streamlined (i.e. hiding the tab bar), while Pulse does it natively and really well. I do have an extension for tab groups that imitates Vivaldi's tab workspaces though, not sure if those will conflict. (If you're curious, it's called Simple Tab Groups.)

Edit: There's also the issue of Mozilla calling the editable userChrome.css a legacy feature, which kinda worries me about its possible removal. Pulse having vertical tabs as a native feature skips that.