Suspicious!! Rated 1 out of 5 stars

Does not appear to work at all with https. Upon closer inspection, the only option in the Add-ons Manager is to "enable marketing". Which is selected by default. A peek inside reveals what appears to be tracking software to gather marketing statistics for advertising or what is commonly otherwise known as "Malware". AVOID this one.
There are others like: https://addons.mozilla.org/en-US/firefox/addon/youtube-high-definition/
that work WITHOUT the tracking.