I'm a long-time fan of the Tree Style Tab extension, but with the coming end-of-life for legacy addons, I decided to try out a WebExtension option. Found this extension via Reddit, and while I agree with previous reviewers that it is still limited (especially the fact that it has zero effect on the native tab bar), as others have mentioned that's more the fault of the in-progress WE protocol than the developers. It does the most important thing, which is let me view all my tabs in a vertical tree.

Just make sure you have the side bar visible, or you won't see your tab tree. Took me an embarrassingly long time to figure that out. :)