Until mozilla gets around to creating the API to hide the tab bar, do this:
Go into about:config and set browser.tabs.drawInTitlebar to false
Go into about:profiles, click Open Folder on root directory, create or open the "chrome" folder and create or edit a "userChrome.css" file in that folder (capitalization counts, even on Windows). Enter the below in the userChrome.css file, save it, close and restart Firefox:
