I've tried simple tab groups before but it was occasionally a bit janky and I was looking for a different UI implementation.
Not really keen on using a sidebar either and the majority of extensions I found all seemed to be using the sidebar.
+1 for tree style tabs. Every time you open a link it is automatically a child of the current tab, so you don't have to manually group everything. (Although you can)
Makes me hate the tab groups in dege/chrome every time (work ☹️) I'm forced to use them.