Mozilla plans to add sponsored content to its Firefox browser in a bid to increase and diversify its revenue stream.

Since the start of the year, the company has been showing some Firefox users links to recommended content on its New Tab page. Some proportion of the recommendations are sponsored, with content producers paying to be included in the list of recommendations. Those links are now also available in the nightly and beta releases. In Firefox 60, due to ship on May 9th, the feature will roll out to all Firefox users in the US.

The recommended links are personalized, with Mozilla saying that the links should be valuable content that's worth taking the time to read. Normally, such personalization raises privacy concerns because effective personalization requires the tracking of personal preferences and habits to ascertain what things a person is likely to be interested in. But Mozilla's personalization is different: it happens entirely on the client side. The browser will download a list of recommended links each day. Each link will also have a list of related websites, with similar kinds of content to that in the sponsored links. The browser will then compare these related sites to your browsing history; if there are lots of matches, Firefox will assume that you're interested in the recommended content and show it to you.

By default, Firefox will track which recommended sites you visit, and how often each recommendation is shown. This is part of the browser's broader tracking of technical and interaction data. Firefox users that don't like these recommendations can disable them entirely. It's also possible to retain the recommendations in general but disable specifically those stories that are sponsored. Advertisers will be told the aggregate click-through rates for their stories but not fine-grained, personal information.

The underlying technology comes from Pocket, which Mozilla acquired early last year. It represents Mozilla's second attempt at earning revenue through sponsored content. In 2014, the company had paid advertisements on its New Tab page in a system called "Tiles." However, these were killed off in 2015, with the company suggesting that the Tiles content wasn't very appealing to Firefox users.

Mozilla argues that boosting the visibility of high quality stories is good for the broader health of the Internet, as in principle the promoted links shouldn't be clickbait and should respect privacy. It's also important for the browser developer: currently, the lion's share of Mozilla's revenue comes from its deal with Google to use Google's search engine as the default. With Google also a competitor to Mozilla thanks to its Chrome browser, the arrangement feels a little precarious. Diversified revenue sources should offer greater resilience.