The horizontal tab bar can be hidden with the following steps:
- 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
- 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.
- 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)
As of Firefox 69: By default, userChrome.css modifications are disabled in Firefox. You need to make sure that on the about:config page in Firefox, the toolkit.legacyUserProfileCustomizations.stylesheets preference is set to true and then restart the browser. (see https://bugzilla.mozilla.org/show_bug.cgi?id=1541233)
EDIT: Added macOS. h/t: @coderuby