The addon lies to you and says stuff like WEBM is impossible to download UNLESS you buy their own MALWARE ridden software !
Also, in Firefox addons, click on the addons OPTIONS, it will say 'File not found' - super dodgy !
The dev is, if not an actual scammer, then someone who doesn't understand the Firefox addon community ethos and is trying to monetize the addon. Greedy b**t**d.