In the latest version of Firefox, after installing Tree Style Tab, the native browser tabs are not hidden.
Check the Firefox version you are using. I’m using 65.0.1.
Firefox version
To find Firefox profile follow the steps given in the link below:
Profiles - Where Firefox stores your bookmarks, passwords and other user data | Firefox Help_Firefox stores your personal information and settings in a profile folder. Find out what is in your profile and how to…_support.mozilla.org
We need to update userChrome.css(can be used to add custom style to Firefox) file to hide the browser tabs.
In your Firefox profile directory create /chrome/userChrome.css and add the following content:
#TabsToolbar { visibility: collapse !important;}
Restart Firefox. (Force quit if needed)
Browser tabs are hidden
There are multiple answers if you google for this solution.
This one mentioned here on reddit seems to be working for me.
Reddit answer by Castratikon seems to be working for me.
One more possible answer can be found here. It suggests to add
#tabbrowser-tabs { visibility: collapse !important; }
didn’t work for me though.
https://www.ghacks.net/2017/09/27/tree-style-tab-is-a-webextension-now/