The maker of the Firefox web browser released its second app for iPhones and iPads on Tuesday as it tries to gain a foothold on Apple's mobile operating system.
The new app, Focus for Firefox, isn't a full browser like Mozilla's first iOS app, Firefox. Instead, it's a plug-in for Safari, Apple's browser, that is supposed to make webpages load faster on iPhones and iPads by blocking ads that track user behavior.
For users, a big draw is that Focus is free unlike many of the top iOS ad blockers. Although a few free options exist, none of them have the backing of a company as big as Mozilla.
However, Focus isn't strictly an ad blocker because it doesn't specifically try to block ads. Rather, it is aimed at stopping ad trackers, the little bits of code commonly embedded in websites designed to collect data about your browsing habits. Ads that don't track you will still show up.
There's a debate about whether content blockers and ad blockers could starve websites of the advertising revenue they rely on to survive. Apple accelerated this debate earlier this year by giving developers the option to create content blockers in the latest version of iOS, its mobile operating system.
By targeting tracking and not advertising in general, Mozilla is looking to find a middle ground. "We want this product to encourage a discussion about users and content providers, instead of monetizing users’ mistrust and pulling value out of the Web ecosystem," Denelle Dixon-Thayer, Mozilla's legal and business officer, explained in a statement.
When users first boot up Focus, they're asked to configure the software before they start browsing. Users can block four types of trackers: ads, analytics, social, and "other." Mozilla warns that blocking "other content trackers" is the most extreme option of the four, and can result in webpages not loading correctly. For example, it blocks embedded videos in certain webpages.
Ad blockers work by checking the requests a browser receives against a "blocklist" of prohibited websites or services. Mozilla has chosen to use a blocklist maintained by startup Disconnect, which is also used to block trackers in Firefox's private browsing mode on desktop computers. The entire list is publicly available on Github, the open source software library. Launching an ad-blocker for iPhones is a natural progression for Mozilla, which has championed Do Not Track and other Internet privacy standards for years, and recently started to build a content blocker into Firefox on the desktop.
Ironically, although Mozilla introduced a standalone Firefox browser for iOS last month, Focus only is only compatible with mobile Safari because of restrictions enforced by Apple. Mozilla says it's "exploring how we can provide this feature on Firefox for iOS and will deliver it as soon as it’s possible."
For more about ad blockers, watch this Fortune video: