For every one looking for a solution to hide the horizontal tabs, tested and approved : https://www.hackerway.ch/2017/11/17/firefox-57-quantum-and-vertical-tabs/

I was using Tab Kit 2nd Edition and very much liked it. With Version 52.0 it went obsolete, so I'm using this now, This is pretty nice, but a couple of things I'd like to see:

1. Resolution to hiding the horizontal tab bar.

2. A true autohide of the vertical tab bar that does not require Ctrl-Shift-V.

This is very nice anyway. Would be great to see these things as well.

Fellow dev here. Nice work - this is great.

Great addition. One of the enhancements you can make to your browsers that makes you go "Why wasn't it always like this?".

One significant improvement would be to allow users to reverse order the list. I open a lot of tabs and tended to work from right back to left. In vertical tabs, this means I am working from bottom to top and I would much prefer to work from top to bottom. Please consider allowing users to reverse the order of the list!

Apart from that, works great 5*

Really clean & useful tab's plugin. Just one litle issue, we cant use it to disable main tab's bar. I know you can't do it with a plugin since firefox 57 but it's sad ;(

after loosing Tab Mix Plus i checked this. its good+works. for me the theme fonts are too small@the moment. and i like tree tabs option to save sessions (should be autosave as well)

Works great, and clean look, as of Nov 28, 2018. I don't see any of the negative issues that some users in older reviews mentioned. Definitely give it a try.

I love everything about it

Good add-on but you can't change the way the tabs look, apart from four (not too good) preset "themes". Moreover, the "themes" override the tab text colours, so "unread", "unloaded" or "read" tabs look the same.
You can set an option to "underline" unread or "strikethrough" unloaded, but nothing more.

Toolbar (new tab and settings) doesn't function, making this sidebar a non-replacement for default tab bar for purely GUI control.

Works great! Firefox 60.3.0esr (64-bit).