Way back in 2014, Firefox rolled out an unpopular feature to its nightly builds: sponsored tiles on its "new tab" page. The feature, which was opt-in by default, showed ads that were based on your browsing history. Eventually, after a very long beta testing period, the sponsored tiles were loosed upon all 500 million-or-so Firefox users in May this year.
Now, just a few months later, the feature is being retired. Sponsored tiles will continue to appear for the next few months while Mozilla "fulfils its commitments" (i.e. clears out ad inventory), but then they'll be gone entirely. Writing on the official Mozilla blog, vice president Darren Herman explains that, "advertising in Firefox could be a great business, but it isn’t the right business for us at this time because we want to focus on core experiences for our users."
Later in the blog post, which was probably published on Friday afternoon in an attempt to dodge the news cycle, Herman repeats the refrain that we've heard many times over the last few years: "We believe that the advertising ecosystem needs to do better ... Mozilla will continue to explore ways to bring a better balance to the advertising ecosystem for everyone’s benefit, and to build successful products that respect user privacy and deliver experiences based upon transparency, choice and control."