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Note that Tab Center will stop working by the end of this year, but I have found https://addons.mozilla.org/en-US/firefox/addon/tab-center-re... to be an adequate replacement!

How is this different from Tree Tabs?

There's no tree, it's flat. I find it works better than Tree Tabs at the moment unless you need the tree.

Shameless plug: the current lack of a WebExtension for vertical tabs bothered me, so I wrote one: https://addons.mozilla.org/en-US/firefox/addon/vertigo-tabs/