Firefox Quantum (ver >= 57) - How Can I Hide the Horizontal Tab Bar with TreeStyle Tabs?

Solution 1:

The horizontal tab bar can be hidden with the following steps:

1. Confirm or Create:

   Linux: ~/.mozilla/firefox/<profile name>/chrome/userChrome.css
   Windows: %AppData%\Mozilla\Firefox\Profiles\<profile name>\chrome\userChrome.css
   macOS: ~/Library/Application Support/Firefox/Profiles/<profile name>/chrome/userChrome.css
   

2. Add the Following code to: userChrome.css

   @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
   #TabsToolbar {
       visibility: collapse !important;
   }
   

   ^Note: Line 1 is required when creating the file and does not need to be included more than once.
3. Restart Firefox

As this is a custom workaround until either Firefox itself allows the horizontal tab bar to be hidden, or the TreeStyle Tabs extension itself hides it, this information may become dated in the future.

Confirmed Testing: Firefox Nightly 58.0a1 (2017-10-23) (64-bit)

EDIT: Added macOS. h/t: @coderuby