Don't forget to right click the addon button and opt out from "sharing data" to keep your privacy. (Thanks God, they have this option).

Give some useful info about sites, #tags, country, webrank, monthly visit, worldwide (but not the visiting countries, or the $$$), country rank. The suggestions of alternative sites is too general. If you visit for instance a newspaper site, it gives you high ranked newspaper sites (that you probably already know), but doesn't give really alternative sites, like the same kind of newspaper, with the same topics, or political orientation. So, it's a general tool for a short analyze of websites. But concerning similarities: oo broad, not really similar... (I wish we could suggest alternative sites instead, or they use narrower tags)

"I installed it, it doesn't appear anywhere and no explanation is given here. I'm going to uninstall it." -> You have to click the extension button!

https://addons.mozilla.org/addon/similarsites/versions/7.1.1/updateinfo/

– empty, no release notes. What's new?