Hi, there is no built-in way to hide the tab bar. The purpose of having it there is when you have multiple tabs open in the window, to be able to switch between them more easily.

It is possible to hide the bar using custom style rules in a userChrome.css file. I don't know whether you've heard of that before. A good place to ask for help with rules is the following subreddit:

https://www.reddit.com/r/FirefoxCSS/

When multiple tabs are open they are already at the bottom of the screen on the toolbar so it seems redundant. A userChrome.css file is probably above my pay grade & too involved so I'll just leave it as is. Thanks