Mozilla Corp. is considering adding a tool to Firefox 3.0 that would automatically block Web sites thought to harbor malicious downloads, but the company's security chief refused to spell out details, saying Mozilla is "not ready to talk about the feature." Even so, there are numerous details to be found on a blog by a Firefox designer and in a back-and-forth discussion of the feature in Bugzilla, the management system that Mozilla uses to track changes in its software. Together, the two offer a behind-the-scenes look into Mozilla's open-source development process.