Tree Tabs (https://addons.mozilla.org/en-US/firefox/addon/tree-tabs/ https://addons.mozilla.org/en-US/firefox/addon/tree-tabs/) is the WebExtensions version of

Tree Tabs (https://addons.mozilla.org/en-US/firefox/addon/tree-tabs/ https://addons.mozilla.org/en-US/firefox/addon/tree-tabs/) is the WebExtensions version of Tree Style Tabs. It works. It's a bit unpolished UI wise, but that should improve. There are two crucial missing bits of functionality in it. One is respecting tab order. Currently it just lets you organize the tabs, but will not actually reorder them under the hood, i.e. the ordering that ctrl-tab respects. What it should do is reorder tabs as they are moved around. Tab Center Redux does this, so I suspect the webextension API exists (in fact IIRC the google chrome webextension tabs API, which Firefox definitely supports, has this), so it just needs the stuff to be fixed. I considered fixing it myself, but the extension doesn't seem to be open source (you can of course inspect the source of the bundle but then there's no easy way to contribute back) This may even have been fixed by the time I looked at it last. The second is that it doesn't actually hide the tab bar up top. Neither does Tab Center Redux. This is a matter of a missing webextensions api, but it's one that's probably easy to add. The other missing bits are mostly minor, like configurability etc.