Firefox still doesn’t have a native vertical tab bar.
At least the extension APIs are powerful enough to have an extension that does a decent job (or even a great job, in the case of extensions like Sidebery), plus there’s a way to hide the regular top tabs. That’s not the case with Chrome - all the Chrome vertical tab extensions feel kinda janky and the regular top tabs are still visible.
You could also use a Firefox fork like Floorp that has native support for tree-style tabs.