Vertical tabs addons have been a thing for years yes. But it is clunky and does not work as well as the native implementation.

Also the notion that Mozilla should "just support that"

lol

This is a thing the devs of Firefox should make and implement.

Supporting working open source implementations of software that are already part of your ecosystem, lol, crazy, right. /s

When have they supported an addon like that officially?

They are making their own solution for vertical tabs.

You are free to install addons.

What is the issue?

Don't just make these snipe driveby comments.

It's a very clunky solution that falls apart the moment you turn on whatever Incognito was named in Firefox.