The testpilot program from mozilla has a beta test for sidebar tabs. This is still not tree, but can be a decent alternative if you need tree style tabs to reclaim screen real-estate.
The sidebar tabs testpilot extension is also not a WebExtension, so it will stop working at the same time as TreeStyleTabs.
Tab Center Redux <https://addons.mozilla.org/en-US/firefox/addon/tab-center-re... is a fork of the testpilot extension that is a WebExtension, so it works fine in Firefox Nightly. It's naturally more limited than the original extension, though: it can't hide the top tab-bar, it has to draw its own context-menu, it can't be shown at the same time as any other sidebar, etc.
It's still nicer than the top tab-bar for those of us with a bunch of tabs open, though.
Tab Center Redux <https://addons.mozilla.org/en-US/firefox/addon/tab-center-re... is a fork of the testpilot extension that is a WebExtension, so it works fine in Firefox Nightly. It's naturally more limited than the original extension, though: it can't hide the top tab-bar, it has to draw its own context-menu, it can't be shown at the same time as any other sidebar, etc.
It's still nicer than the top tab-bar for those of us with a bunch of tabs open, though.