For anyone having trouble with the horizontal tabs not hiding, Anonymous user 099626's advice worked for me. Create a folder chrome/ in your firefox profile and then in that directory a userChrome.css file containing the following rules

/* Remove horizontal tabs */
#tabbrowser-tabs {
visibility: collapse;
}

If you want to get rid of the sidebar header, add this to the file:

/* Remove sidebar header */
#sidebar-header {
visibility: collapse;
}

You will have to restart Firefox to enact the changes.

Best extension ever that keep me using Firefox

Must have дополнение для ФФ. Пользуюсь уже много лет.

ツリー型タブのアドオンを1年前ほどからずっと入れてましたがFirefoxのアプデにより
ツリーの固定化と最大縮小サイズが狭くなってしまいもっと狭く設定できるようにしてほしいです。
このアドオンを使わないと生きていけない体になってしまい、とても良いアドオンだと思います

Practically essential if you open many tabs. Hierarchical indentation is what makes this superior to all other solutions.

The transition to Web Extensions is excellent considering the situation. There is a current bug on Firefox's end that keeps the horizontal tabs around. You can address this by creating a file in your Firefox profile, \chrome\userChrome.css and add the following rules:

/* Remove horizontal tabs */
#tabbrowser-tabs {
visibility: collapse;
}

If you want to get rid of the sidebar header, add this to the file:

/* Remove sidebar header */
#sidebar-header {
visibility: collapse;
}

You will have to restart Firefox to enact the changes.

I loved having the autohide tabs on the side and no tabs at the top. It was just easier for me when I was search for something, say Thumb drives, and I could easily locate the store page from all the listings I was looking at. The autohide tree tabs on the side gave me more screen space both vertically and horizontally and I miss it sorely. I hope to see this updated soon for ff57!

Auto hide not working in FF57 & display as sidebar! And horizontal tab too displayed.

My favorite extension. Works great with Firefox 57+. It has many options and also allows customization via CSS (I use it to increase font). It doesn't hide top tab bar because of API limitation but for me it is feature not bug :) because I sometimes use also top bar for example when I need more horizontal space.

The reason why I continue using Firefox. Viewing pages in a tree is a great way to keep track of my day-to-day projects.

Such a lovely way to view tabs!

Great way to organize tabs if you're a million-tab user like me.
One request (and I know it's early) - in Firefox Quantum, the tab bar at the top stays! So now I have 2 sets of tabs - the normal Firefox tabs, and the tree style tabs. Would be great if there was a way to hide the tabs at the top. Thanks!

This is the best extension ever. I would also like to be able to hide the tabs at the top in Firefox 57.0.

Can't imagine browser without this addon.
The main reason why I'm still on FF.