In a talk delivered last Wednesday at the Mozilla Summit in Whistler, Canada, Pascal Finette, director of Mozilla Labs, asked an audience of more than 150 Web developers a hypothetical question: what would an “open” Web app store look like? The answer could play an important role in the future of personal computing.

The success of Apple’s app store, as both a consumer phenomenon and a new stream of revenue, has changed the software landscape–but only on mobile devices. Now the race is on to adapt this model by developing an app store for the Web itself. In a Web app store, developers could sell applications that would run on any device with a Web browser, independent of hardware or the operating system.

Google announced plans in May to create the Chrome Web Store, which will integrate directly with its Chrome browser. Microsoft also recently began offering Web app versions of its office software.

Also in May, the Mozilla Foundation, a nonprofit organization that provides the Firefox Web browser and other software, revealed its own plans to build a Web app store–one that it promises will be more “open” than anything else. An open strategy-one based on open standards and unrestrictive licensing–has helped the Firefox browser gain market share from Microsoft’s Internet Explorer, and the Mozilla Foundation is betting that the same approach will attract users and developers to its Web app store.

Mozilla’s vice president, Jay Sullivan, also laid out the principles of an open app store in a blog post last May. The first principle, Sullivan argued, is that an open app store must host only applications based on open standards, including HTML5, CSS, and Javascript. This would preclude apps that use proprietary technologies like Flash and the Unity 3D graphic plug-in. (Although perhaps not everyone agrees with this requirement. Google’s announcement of its Chrome Web app store included a demo of the Lego Star Wars game, which includes the Unity plug-in.)

Sullivan also argued that an open Web app store must work equally well across all browsers, should be accessible to all developers, and should not gather user information. Finally, he said, it should have transparent app review guidelines–which would distinguish it from Apple’s iPhone and iPad app store.

A few specialized Web app stores already exist. For example, in 2006, Salesforce.com, the enterprise customer relations management platform, launched its own app store, now called the App Exchange.

App Exchange has more than 400 native apps running on Salesforce.com’s own platform, as well as more than 1,000 third-party apps that integrate with the Salesforce.com platform. It might not sound like a large library, but according to Chuck Ganapathi, senior vice president for products at Salesforce.com, those apps have been downloaded or test-driven 350,000 times, and all of them are “serious business apps,” rather than the entertainment that dominates most app stores.

Providing a way for users to discover new software is usually cited as the primary reason for developing an app store, but Ganapathi says a well-run app store has benefits beyond acting as a central location for users to find apps. “Security and privacy are also a big concern,” he says. “IT organizations [like the ones that use Salesforce.com] are far more concerned with this than consumers, so we had to deal with this earlier.”

Security is one of the main reasons apps must undergo review before being accepted by Apple’s app store. But the review process has also been a point of friction between developers and Apple, because so many apps get rejected. In contrast, the Android Marketplace, the app store for Android-based devices, has no oversight other than user ratings and comments, which means potentially malicious or buggy apps are only reviewed if they appear to cause problems.

“I personally think there is some sort of middle ground here where you can leverage the collective wisdom of crowds to do some of the vetting [of apps] for you in a more open and transparent way,” says Mozilla’s Finette.

For developers, the ability to monetize apps is a big motivator, although it’s unclear how easy it will be to make money from consumer Web apps. But in contrast to current subscription-based Web services, Web apps appear in a privileged location in the browser, and may even be downloadable. Google has announced a downloadable function for its Chrome app store, and the HTML5 standard also provides for caching of local data, even entire apps–so that they work offline, like regular software.

Finette notes that Web apps might not even need to live in the browser–using technology like Mozilla’s Prism, it’s possible for Web apps to live on the desktop and launch without the usual URL bar and the forward and back buttons that normally surround a Web page or application.

Consumers may be drawn to the fact that Web apps wouldn’t be locked in to a particular platform or operating system. “I spoke to one person who said, ‘I spent $250 in applications on my iPhone–if switched to Android, I would throw that [money] away,’” says Finette. Web apps could work on any device with a standards-compliant browser, letting users change platforms without losing their investment.

Web apps also promise to make life easier for developers, who would no longer need to rebuild their programs for each new platform. The iPhone and Android phones run apps written in different languages, and it is an even bigger challenge to port the same app between Windows, OS X, and Linux. All of this means that “transition costs for developers are significant,” says Finette.

Google declined to comment on its forthcoming app store beyond its previous public announcements, so it remains to be seen how the Chrome app store will compare to Mozilla’s vision or whether the two might even be compatible. Ultimately, Finette will have to wait for developers, and users, to answer his question.