[quantummkv]

Unless I am mistaken, the extension never collected any user data and phoned it to a nebulous ad network. So user data still remained private

[nbst]

When a request is made to a server, the server creates a log of it. When you interacted with the site that the plugin worked in tandem with, it requested assets (JavaScript, images, CSS, etc) to Firefox-run servers. That's enough for them to know you looked at that content and may have interest in it. Not a huge deal in that particular instance but it's CERTAINLY not putting the user's privacy first.