AddOn:
- Go to about:support in your address bar
- Look for your profiles directory and open it:
- Create a file named
chrome/userChrome.cssin your profile directory:
mkdir chrome && touch chrome/userChrome.css
- Populate the following css code:
#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {
opacity: 0;
pointer-events: none;
}
#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
visibility: collapse !important;
}
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
display: none;
}
.tab {
margin-left: 1px;
margin-right: 1px;
}
- Then go to about:config in your address bar and set this to
true:
toolkit.legacyUserProfileCustomizations.stylesheets

There's now a helper extension for Tree Style Tabs that provides a button to toggle this (for both MacOS and Windows):
https://addons.mozilla.org/en-US/firefox/addon/toggle-native-tab-bar/
It walks you through how to set up the folder and file inside your profile properly and provides a button in your browser that, once set up, lets you turn native tabs on and off
In fact, there's a BUNCH of handy helper extensions that build upon Tree Style Tabs to make it do more stuff:
https://github.com/piroor/treestyletab/wiki/Helper-addons-extending-functionality-of-TST