Version 3.0.0 does NOT work anymore since last update (of Mar 16, 2021). Sometimes things like that happened so you just have to download the previous version BUT the developer STUPIDLY REMOVED all previous ones in version history. It's why I'm giving 1 STAR ONLY.

***UPDATE*** Guys, instead use the extension called: "YouTube Video and Audio Downloader (WebEx) by feller". It works like a charm AND it's recommended by FireFox