How to hide tab bar (tabstrip) in Firefox 57+ Quantum

How to hide/disable/remove the tab bar in Firefox 57+ ("Quantum")?

The goal is to just hide the tab bar totally. Useful if paired with extensions like "Tree Style Tab".

Note: there is similarly looking question here: Firefox hide everything except content area of the browser but it is not properly split into logically independent tasks. Also, it's outdated.

1. Open your firefox "profile directory"
   • Official docs how to do that: https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_how-do-i-find-my-profile
   • Or just open:
     • Linux: ~/.mozilla/firefox/<profile>
     • OSX:
       • ~/Library/Application Support/Firefox/Profiles/<profile folder>
       • or ~/Library/Mozilla/Firefox/Profiles/<profile folder>
     • Windows: %appdata%\Mozilla\Firefox\Profiles\<profile folder>
2. Create directory chrome/ if it doesn't exist
3. Create file chrome/userChrome.css inside if it doesn't exist.

4. Add this text to the file:

   #TabsToolbar { visibility: collapse !important; }
   

5. Ensure the config toolkit.legacyUserProfileCustomizations.stylesheets is set to true (required for Firefox69+, the stable version since September 2019), see this tutorial.

6. Save the file and reload firefox. You should see no tab bar anymore.

P.S. Solution partly taken from here: https://www.ghacks.net/2017/09/27/tree-style-tab-is-a-webextension-now/

I want the tab bar to auto hide when there's 1 tab and appear when there's multiple. Not the same as the question but this is about the only Google result right now for 57+ so for those who need it in userChrome.css

#tabbrowser-tabs, #tabbrowser-tabs arrowscrollbox { min-height: 0 !important; }
#tabbrowser-tabs tab { height: var(--tab-min-height); }
#tabbrowser-tabs tab:first-of-type:last-of-type { display: none !important; }