Is this allowed? Shouldn’t the main addon be banned from AMO? At least, this is very bad style and I don’t think this kind of addon should be allowed by Mozilla!
P.S. The addon is question is: https://addons.mozilla.org/de/firefox/addon/youtube-unblocker-plus
You can safely install the main addon, then watch the website which opens autmatically. Don’t click anything there! Instead, analyze the source code of the website and linked “support addon” to verity what’s going on. (You may use Google Translate for this website)