Mozilla has ended its ambitious entry into the smartphone race, saying Tuesday it will stop offering Firefox OS phones through operators.

The withdrawal is unsurprising, even though Mozilla heralded the OS as one of its major projects in its recent annual report.

Some operators shipped Firefox OS phones, which were designed to be lower-cost alternatives to Android-powered ones and Apple’s iPhone, but the devices never gained significant market traction.

“Firefox OS proved the flexibility of the Web, scaling from low-end smartphones all the way up to HD TVs,” according to a statement from Ari Jaaksi, Mozilla’s senior vice president of connected devices.

“However, we weren’t able to offer the best user experience possible, and so we will stop offering Firefox OS smartphones through carrier channels.”

Mozilla’s change of plans illustrates just how difficult it is to develop competitors to Android and Apple’s iPhone, which dominate smartphone sales.

It was likely particularly disappointing for Mozilla, whose Firefox desktop browser, which debuted in 2002, shot to popularity and spurred major, positive changes to web browsers.

Mozilla’s plan was to move away from the app store models and instead optimize Firefox OS for HTML5, the latest specification of the Web’s mother tongue.

HTML5 has features in it that allow interactive apps delivered through the Web rather than standalone apps. It’s the future of the Web for desktops, but mobile is still a different game.

For the Firefox OS to be successful, app developers would have had to embrace the platform more enthusiastically. Although in theory any HTML5 Web app works on a Firefox OS phone, it’s not that simple. The apps would still need to be fine-tuned to use mobile hardware such as gyroscopes and cameras.

Robust demand for Firefox OS devices might have motivated developers more, but that didn’t happen, despite models shipped by operators including ZTE, Telefónica and Alcatel.

Firefox OS is an open-source project, so it is possible part of it will live on in some form. Mozilla said its Firefox OS team would remain intact and “continue to work on the new experiments across connected devices.”

Firefox OS borrows much from the Firefox mobile browser and Gecko application framework, which is used to render Web pages and display applications. The platform underpinning Firefox OS, called Boot to Gecko (B2G), borrows 95 percent of its code from the mobile browser and Gecko.

The mobile OS uses a Linux kernel, which then boots into the Gecko runtime. The top layer of the technology stack, called Gaia, generates the interface seen by users.