Mozilla recently launched a “content blocker” for iOS 9. In spite of the name, the app is actually designed to only block ads that are trackers.

The new app, called Focus by Firefox, takes advantage of iOS 9’s content-blocking feature. Users of the app will also need to access the Settings feature of iOS to set up content blocking. Fortunately, the app provides users with a step-by-step configuration guide.

Even though the app uses the content-blocking feature of iOS 9, it doesn’t block all ads. Instead, it just blocks ads that trackers.

Trackers are ad units that collect data about a user's activity across multiple websites or apps, some of which might not be owned by the data collector. Trackers also aggregate and share the data collected.

“This is our first effort,” says Nick Nguyen, Firefox’s vice president of product. “Just like everything we build, we’re trying to make the Internet better, long-term.”

Nguyen added that many other iOS 9 blockers are for-profit. Focus, on the other hand, is free.

Users can also control which types of trackers to block. The app categorizes trackers into various categories: analytics, social, ads, and “other content trackers.” The “other” option essentially blocks everything, and may prevent some websites from loading properly.

“The other content trackers category includes things like video, photo and embeddable content that tracks users,” Nguyen says. “Turning this category on will block some video embeds and may even break some sites completely.”

Focus also gives users the option to block Web fonts. That improves the mobile experience by reducing data usage and reducing load times.

The app employs the blocking list produced by Disconnect and published under the General Public License. That list is available for anyone to view on GitHub.

“We think Disconnect’s public list provides a good starting point that demonstrates the value of open data,” says Denelle Dixon-Thayer, Mozilla’s legal and business officer. “It bases its list on a public definition of tracking and publicly identifies any changes it makes to that list, so users and content providers can see and understand the standards it is applying.”

An important point to remember for iOS users, though, is that the app only works with Firefox. Users who prefer to visit websites using the operating system’s native browser, Safari, won’t see ad content blocked.

“This was not our choice — Apple has chosen to make content blocking unavailable to third party browsers on iOS,” Nguyen says. “Apple Developer Guidelines do not allow us to incorporate their Content Blocking API into Private Browsing on Firefox for iOS. We would love to see this API open up in the future. We are exploring how we can provide this feature on Firefox for iOS and will deliver it as soon as it’s possible.”

Nguyen says that he’d also like to build the new app “into anything that runs iOS 9.”