Firefox 57+ Treestyle Tab

README.md

Firefox users who want to hide the tab bar at this point in time may add the following line to the userChrome.css file in the /chrome/ directory of the Firefox profile to do so:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* to hide the native tabs */
#TabsToolbar {
    visibility: collapse;
}

/* to hide the sidebar header */
#sidebar-header {
    visibility: collapse;
}

Set Firefox to look for userChrome.css at startup (updated 5/24/2019)

To make startup faster for most users, Firefox 69 will no longer look for this file automatically. You need to tell it to look. Here's how:

• In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
• In the search box above the list, type or paste userprof and pause while the list is filtered. If you do not see anything on the list, please ignore the rest of these instructions. You can close this tab now.
• Double-click the toolkit.legacyUserProfileCustomizations.stylesheets preference to switch the value from false to true.

Reference:

• https://www.ghacks.net/2017/09/27/tree-style-tab-is-a-webextension-now/
• https://www.userchrome.org/how-create-userchrome-css.html

You may have to enable the Title Bar to get rid of the window close button misalignment.