You might want to check out Test Pilot and the Tab Center addon : https://testpilot.firefox.com/experiments/tab-center

Unfortunately it is very slow compared to Tree Style Tabs if you have a considerable amount of tabs open (even using extensions like auto-unload, it is very very slow). Hope it gets fixed because I would _love_ a built-in vertical tab feature.

I have hundreds of tabs open, and use Tab Center, and I haven't experienced it being slow.

However, past a number of tabs (enough to fill the "tab bar" vertically), it doesn't show thumbnails, maybe that's the difference?

I used to have those kinds of hundreds of tabs in multiple windows until I found Onetab: https://addons.mozilla.org/en-US/firefox/addon/onetab/

Still using Tree style tabs for their sub tabs feature which helps keeping things organised for me.

On my machine (Intel Celeron, 8g RAM) it's noticeably slower than TST even with only 50 or so tabs

I'm using nightly version 53 and Tree Style Tabs works fine

Using nightly here and it's kind of buggy. New tabs don't appear unless I click on them, and the very first tab is overflowing on my bookmark bar

I'm on tree styles tabs 0.18.2016090802 and firefox 50.0 and haven't had any problems.