The Mozilla Foundation on Tuesday unveiled two preview smartphones as it invited developers to try its new open-source mobile operating system challenging Apple’s iOS and Google’s Android. The announcement marked a major step forward for the new Firefox OS mobile operating system which is being built using open Web standards, like its Firefox Web browser. The two preview phones being offered come from the small Spanish-based manufacturer GeeksPhone, the first in what the nonprofit group hopes will be a series of low-cost smartphones which can be sold around the world. “This week we are announcing our new Firefox OS developer preview phones because we believe that developers will help bring the power of the Web to mobile,” said a blog posting from Stormy Peters, head of websites and developer engagement at Mozilla. The developer phones are being made by GeeksPhone in partnership with the Spanish carrier Telefonica. “If you’re a developer interested in Web technologies and mobile, now is the time to try out Firefox OS,” Peters said. The operating system, she said, is an effort to “keep the Web open” and “help make sure the power of the Web is available to everyone — even on mobile devices.” By using the open platform, she said, “you’re not locked in to a vendor-controlled ecosystem. You can distribute your app through the Firefox Marketplace, your own website, or any other store based on Mozilla’s open app store technology.” The non-profit group’s so-called Boot to Gecko project will go after Google’s Android or Apple’s iOS, to create an alternative which could generate smartphones that are less expensive than an iPhone while offering similar experiences to those running on other platforms. GeeksPhone said on its website that the two new phones were named Keon, with a 3.5 inch display, and Peak, with a larger 4.3 inch screen. Both will use Qualcomm Snapdragon chips.