While much open source software suffers from poor design and usability, Firefox shines. What makes the Mozilla community different? With great branding, usability backed up by research but tempered by realism, and a powerful extension architecture, the Firefox web browser claims 400 million users. On the eve of the release of Firefox 4, Mozilla designer Alex Faaborg covers the unique challenge of coordinating user experience design in an open source community, important features of past versions, and the future of the Firefox interface.

Alex explains that Mozilla's mission is to protect the free and open web. He cites Mozilla founder Jamie Zawinski who fears the web "could all turn into television again," with exclusivity, lock-in, and widespread monetization. Ordinary people have learned to expect exclusivity and lock-in, as Alex learned from a cab driver who told him he decided not to switch to Firefox because he "really likes YouTube."

But Alex clarifies that Mozilla contributors aren't communists: He observes, "The web is the freest market humanity has ever seen," enabling the growth of billion-dollar companies in just a few years.

In addition to coders, Mozilla benefits from contributors in marketing, the law, and, of course, design. Why do all these people freely contribute? Among other reasons:

• Passion and glory.
• Access to other highly skilled contributors.
• Because their work on Mozilla is part of their self-identity.

Design in an open-source project can be chaos, as everyone wants their idea included. Alex describes a spectrum of design philosophy, from user-centric design, driven by studies and focus groups, to the "strong designer" who knows what users want better than the users themselves. Mozilla succeeds by starting with the strong designer, applying research-based design principles, and then verifying design decisions through user-centered techniques.

Mozilla has several ways to enable diverse design contributors without descending into chaos:

• Educate contributors and focus on core principles: Mozilla's ticket system has a built-in link to a taxonomy of accepted design principles, to ease discussion and consensus-building.
• Contributors have the freedom to explore their ideas: Mozilla's powerful extension architecture was created to enable contributors to implement their ideas, even those that didn't make it into the core product.
• Elevate contributors who know what they're doing.

Alex goes into detail on how design principles are applied, giving many examples from the development of Firefox 3 and Firefox 4.