Useless extension. No idea why Mozilla is advertising this garbage. This extension does only one thing as far as I can tell, it prompts you to download a standalone program. The program is what does the actual downloading. So what does the extension do besides (probably) spy on you and tell you to install that other program?

There's PLENTY of programs that you can use to download youtube videos without needing to compromise your browser privacy and security by using an extension. And god only knows what data the standalone program is sending the developer of this thing.