I use very good addon in Opera for Youtube settings, almost all-in-one (block autoplay, block next videos, block comments, adds download button etc). It is called Magic Actions. It is available in Opera and Chrome official extensions stores. But for FF it is available only from their site and when I clicked it just downloaded the extension (.xpi file). The addon claim it is reviewed and approved from FF, Opera and Chrome yet it is not in official FF addons gallery. Is it safe I to install it or test it before that and if it is safe how to install .xpi file?