I've been using this extension for years and years. It's probably the main reason I stay on Firefox, it's that good. It means I can eaily manage many tabs with ease.

This extension is not just a powerful research tool, but it's surely one of the best additions to internet browsing ever.
Chrome has tab groups, a way of grouping tabs to categorize them. When switching to FireFox I was saddened by a lack of tab groups, but this extension has basically solved that problem.
This extension allows you to have the same effect as tab groups (tabs sorted under 1 thing, as a set) but allowing for tabs to be under other tabs indefinitely, this opens up massive possibilities for tab organization.

For research, it makes it so much easier to sort out tabs, by showing where they came from & allowing for tab trees to be collapsed.
For browsing, opening new tabs & going down internet rabbitholes is much less erratic, since your able to see which tabs came from which, instead of there just being dozens of webpages of unkown origin.

Edit: (18 October 2022) I don't use tree style tabs on firefox. I use firefox so I can have tree style tabs.

Edit: (19 October 2022) It would be nice if there was a style option that displayed guidelines(like literal lines) to show which tabs were childed to which. I also suggest that there be a save session button which saves every single tab & not just 1 tree. (you can work around this by childing all your tabs to another tab, & then just bookmarking that tree, but it's a clunky workaround)

Edit: (2 November 2022) Is it possible for this extension to modify how tabs are seen on mobile devices. Since firefox lacks tab grouping, it can be especially difficult to track tabs on mobile since you can't see them all the time.
If mobile tab view cannot be modified with extensions, that would be sad :( .

I somewhat recently switched from Edge and had fallen in love with vertical tabs, more specifically an auto-hide favicon'd list that expands when hovered over. This addon was able to fulfill all my vertical tab desires ❤️

More details on their GitHub, if you're like me and want a thin sidebar that expands when moused over: https://github.com/piroor/treestyletab/wiki/Code-snippets-for-custom-style-rules#auto-showhide-sidebar-by-mouseover-hover

Really helps with worflow.

Indispensable. There used to be a minor annoyance but they fixed it.