This was a mod to make Firefox look more like the new Safari Unveiled at WWDC 2021. It features tabs and address bar in a unified bar,similar to Safari for MacOS Monterey.
I love Apple's Design , but their pricing ? Not so much.
(Note:All the buttons are customizable and can be removed.)
With few more buttons removed :
To apply this theme to your computer, paste the userChrome.css file to a folder named "chrome" in your Firefox root directory.