Ah right, we're talking different definitions of "Firefox users". I meant that they're not collecting data on specific users, i.e. there's nothing on Mozilla servers that says anything about me specifically. The post is talking about Firefox users as a collective, i.e. "this many Firefox users are searching for animals". Which is something it's done for ages, albeit not for what websites people are loading. (But it is known, for example, which menu items are most used.)
I'll also note that that post is not about advertising but about what features to develop, but I'll grant that it's not a big leap to use it to serve more granular advertisements as well.