When you visit websites, many advertisers store text files on your PC or phone that are designed to show you ads you're more likely to click on. For example, you might check out a product on Amazon then see that same product advertised a later on a different website when web ad software sees you expressed interest.

Firefox Focus blocks that and also pares away traditional interface elements like menus and tabs to speed up browsing. It doesn't go as far as actually blocking ads, though, like Brave or Opera can -- and Google Chrome won't.

Mozilla's move underscores the tension between advertisers and people using the Web. Sites like Google search and Facebook social networking are free because of advertising -- but ads can invade privacy, slow performance, encroach on your monthly data-transfer limits and even deliver software that launches online attacks.

"Much of what makes mobile web pages slow is the technology used to track users on the web. Because Firefox Focus blocks these trackers, it is likely you'll notice a performance boost on the many sites that track your behavior," Mozilla said of the new browser. "When you occasionally see a site that doesn't work because it is dependent on tracking, and if you don't mind that kind of tracking, Firefox Focus makes it easy to open your current site in either Firefox or Safari."

Mozilla pushed a technology called Do Not Track that would have let people tell websites they didn't want publishers and advertisers to keep tabs on them, but the effort fizzled after advertisers objected.

Mozilla already has built tracking protection into Firefox's private tabs on personal computers and mobile devices powered by Google's Android software, but Apple-imposed limits prevented the company from adding that to Firefox for iOS, said Nick Nguyen, vice president of product at Firefox. A workaround led to Firefox Focus.

"One of our engineers built an experiment that was a simple browser on top of our Focus by Firefox content blocker," Nguyen said. "As we lived with the experiment, we found that having a private and lightweight browser with easy controls could be very handy so we are delighted to be releasing it to the public."