Precisely. Firefox is the only real opponent to Chrome (and Google is evil) and have just a few percent market share. So whats the matter, Mozilla needs money to develop Firefox and as long nobody wants to pay for, it has to be an ad from time to time. Also I don't think this ad is intrusive or anything. I would't like it if they sell my browsing history without my consent to make money, this would be newsworthy.
By the way, is it possible that Linux users are treated different? Since I didn't notice this "ad" at all, and as example DRM is under Linux disabled by default, the kb-article is a different one for Windows and Linux: https://support.mozilla.org/en-US/kb/enable-drm#firefox:linu...