I haven't been able to live without vertical tabs in years. This is the best Firefox implementation of it I have found yet.

Just got a big update that improves a bunch of stuff that was already good.

I just discovered that clicking on the tab you're already on switches to the previous one you visited, so repeatedly clicking the same tab switches between the two every click. I have no idea what I'm going to use this feature for, but it feels well designed and I love it.