Mozilla’s mission since its outset in 1998, first as a software project and later as a foundation and company, has been to provide open technology that challenges a dominant corporate product.

Microsoft and its Internet Explorer browser was Mozilla’s original target. Today, Mozilla sees the rise of Apple and Google in mobile software as a “threat to the open Web,” said Jay Sullivan, vice president for products at the Mozilla Corporation.

In recent months, Mozilla has been developing its own open-source smartphone operating system, Boot 2 Gecko. Its published product road map says Boot 2 Gecko will be demonstrated in products before the end of March.

Mozilla is expected to announce handset and carrier partners at the Mobile World Congress in Barcelona, Spain, which starts next week. Brendan Eich, Mozilla’s chief technology officer, said in a recent Twitter message that Boot 2 Gecko would be appearing at the conference “with partners.”

But this week Mozilla announced a step in its smartphone strategy, Mozilla Marketplace, which will begin accepting Web-based smartphone apps next week.

The marketplace, where smartphone developers can offer apps, is intended to make writing Web apps for smartphones pay off. And that, of course, is a crucial ingredient in becoming an alternative to the online app stores of, most notably, Apple and Google. (Google’s Android uses open-source software, but its restrictions on developers still make its software and store a “vertically integrated silo,” Mr. Sullivan said.)

The goal, Mr. Sullivan said, is to “give developers freedom without gatekeepers.” And in the long term, he added, “We think it’s inevitable that there will be an open alternative to these closed ecosystems.”

Perhaps, but challenging the likes of Apple and Google promises to be difficult, though the industry is still young. Even Microsoft, with all its resources and its partnership with Nokia, faces an uphill struggle.

But there is an opening. Many smartphone apps, Mr. Sullivan noted, are already written mainly in HTML5, the latest version of the language of Web pages. But to put them in the Apple or Google world, developers wrap a layer of iOS or Android code around the app, Mr. Sullivan said.

By contrast, the Mozilla approach, Mr. Sullivan explained, is to add a small bit of “metadata” to a developer’s work that signals “this Web site is actually an app.” And the Mozilla software allows a Web app to install an icon on a phone screen, like other smartphone apps.

Mozilla, Mr. Sullivan said, expects that many marketplaces for mobile Web apps will eventually be created, by handset makers, carriers and others.

The Mozilla plan is a bet that the Web software, HTML5, is becoming mature enough to deliver the performance of apps tweaked for iOS or Android, for example. Mr. Sullivan sounds confident. Perhaps not 3-D game programs, but “about 80 percent of mobile apps today can be built with HTML5,” he said, including most media and entertainment programs.