Last year, Mozilla announced that they’re developing native vertical tabs for Firefox, and that it’s available in the Nightly release channel. Since then, despite generally keeping up with things, I hadn’t seen any more news. Turns out, if you look at the bug tracker, they’re still actively working on it.
But it works fine, and you can switch it on with an about:config flag in Firefox stable.
To do so, open a new tab, type about:config and press enter. Ignore the scary warning and search for sidebar.verticalTabs, and double click it to change it from false to true.