If you're like me and primarily used this add-on to remove the tabs from the top, follow these instructions provided by user Soulyaris:

"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 and restart firefox. Enjoy."

Really great add-on. I'm sad to see one of its core features get changed. I hope it gets fixed.

This extension is the only reason I still use Firefox. Something like this should be built into the browser really. Big thanks to the developer and for the effort made for it to work with the new Quantum (57) version. Required some tweaking to make it look the way I like but now works perfectly.

Make sure to visit the addon wiki for tips how to make the most of it (https://github.com/piroor/treestyletab/wiki).

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.

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 :)

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 !

Extension is very handy. But last update asks for accessing all my web data. Why??? I'm not installing it for now cause there is no justification anywhere why this permission has been requested.

To add on to Anonymous user fc7bf5's comment. If you want to get rid of the Horizontal Tabs and the Title Bar for the sidebar simply add the following in your Firefox Profile. This is not the Local AppData one, but the Roaming one which is normally found at: %UserProfile%\Appdata\Roaming\Mozilla\Firefox\Profiles

You will have a profile folder here, maybe more if you utilize multiple profiles in Firefox. Once you pin-point yours you simply need to add a new folder called "Chrome" in your profile folder and create a new text file "userChrome.css" make sure it is a css file and not a txt. you may have to google how to change file extensions by revealing them in Windows. The contents of that file should contain one or both of the following segments depending on what you want.

/* Contributed by Anonymous user 099626's */
/* Remove horizontal tabs */
#tabbrowser-tabs {
visibility: collapse;
}

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

Once this is done you will have to restart FireFox. Depending on your theme in FireFox you may notice you can't see the windows control buttons in the upper right. To fix this simply go to the "Customize..." menu and check the "Title Bar" option then click done.

ps. I've noticed some FireFox menu's don't take the removal of the Horizontal Tabs into account due to it not being a build in option. With this in mind option screens like "Customize..." may take over your full screen including the TreeStyle Tabs. If this happens simply look for another way to close the screen, which should be provided.

pps. TreeStyle Tabs is a fantastic app that I've used for years, I've slowly started moving to Chrome because of the noticeable speed difference and the large amount of multitasking I have to do daily. I am excited to see if FireFox Quantum can live up to the speed it boasts. That being said...

Happy Tab Hoarding! :D