Скрываем вкладки сверху/How you can hide tabs on top
Russian:
Идем в папку профиля фаерфокса. Обычно это лежит в c:\Users\*user*\AppData\Roaming\Mozilla\Firefox\Profiles\*что-нибудь*-default (смотрите по дате измения). Заходим, создаем папку chrome, в ней сосздаем 2 файла userChrome.css и userContent.css(достаточно первого). В файле userChrome.css пишем:
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
/* Следующая строка будет прятать вкладки*/
#TabsToolbar { visibility: collapse; }
Сохраняем, идем в about:config, ставим toolkit.legacyUserProfileCustomizations.stylesheets в true, перезапускаем огнелис. Радуемся.
English
Inside your profile folder(it usual c:\Users\*user*\AppData\Roaming\Mozilla\Firefox\Profiles\*something*-default (check date of modified), create a new folder named chrome. Inside the chrome folder, create two new files, userChrome.css and userContent.css. At the top of userChrome.css, add the following code:
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
/* Hides tab bar*/
#TabsToolbar { visibility: collapse; }
Save it, check toolkit.legacyUserProfileCustomizations.stylesheets in about:config(in FF) is true, and restart firefox. Enjoy.

I'd like to try this, but can't... attempting install tells me that my FF 57 is really 48, and refuses. This is on 64-bit Debian, in case that matters.

What's with the permissions requirement?

This add-on is amazing. Instant fan.

Fantastic extension that helps me manage all the tabs I need to have open everyday. However, FireFox 57.0 update was the worst thing that could ever happen. The extension is not working as it used to and the worst, I have to deal with the traditional horizontal tabs that FireFox offers.

Come on guys, new Firefox 57 has been just released, of course the Tree-style can have some bugs, wait a little.
I'm very grateful to the author for the fast adaptation of the addon to the new FF.
Tree-style was, is and will be must-have even with some bugs.
I'm going to donate.

For those concerned about one of the most recent TST updates (I believe it's 2.2.8) that requires your permission to "Access your data for all websites", note that Piro addresses this concern in issue #1536 on TST's github project, specifically in the following comment: https://github.com/piroor/treestyletab/issues/1536#issuecomment-344792173.

I was concerned about this permission request at first, especially since I have enjoyed TST for so long and didn't want to stop using due to permission overreach. However I think I can accept the risk based on Piro's explanation of why Firefox issues the warning.

Thanks for the great add-on, Piro! I really appreciate your hard work :)

После внезапного краха всё дерево было утрачено. Теперь нужно создавать с нуля. С Tab Groups такого никогда небыло.

Must have extension! I can not work without it. However, last days, very annoying bug. New tabs are not visible in tree sometimes. They are only in standart horizontal title bar.

Can't imagine my browsing experience without this extension. Has been using it for many years. Must have, best of class!

I already use it for long time. great for my 800-900 tabs.
firefox update to 57, with no warnings...
firefox say TST is the same and took my session manager.
now all my trees become flat !!
I tried to use the migration cmd tool on the last session file, but it give access denied and also the exe can not be erase from my folder :-(
I also does not understand all the config in the advance section of this add-on.
just to mantion, it keeps new trees as created.
I could not find firefox 56 download, in order to downgrade.
how can I get all my tree structure back ?
TST 2.2.8
thanks

Awesome.

One of the most useful extension and the number one reason why I use Firefox.

Really nice Add-On, especially for the fact that I can edit the CSS inside the settings.
Just one thing: Would it be possible to add an option to set the animation speed or transition time?

Using it for a long time now... I love it... Firefox should have vertical tabs by default, its way more intuitive to scroll up/down than to move the tablist left/right, and its more productive since pages are vertical usually and vertical space is more precious nowadays

The tree feature is the best grouping/tracking of set of pages

The only thing this addon lack is the coloring of the tabs, would love to see that feature in a future release

To change the size of the tabs do:
/* Change tab height */
.tab {
height: 24px;
}

Following the message of XenGeek
I used this code to remove Tabs keeping the title bar.

/* Remove horizontal tabs */
#tabbrowser-tabs {
visibility: hidden;
margin-top: 40px;
margin-bottom: -50px;
}

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

Anyway this add-on is very good, I don't understand why this tree tab menu is not a standard in Firefox. Saved me a lot of time and energy !

Thanks a lot !