A Bugzilla/Testopia developer works on two interconnected systems. Bugzilla is a web-based general-purpose bug tracking system, while Testopia is a test case management extension for Bugzilla. The developer's responsibilities include creating, modifying, and maintaining these applications to help users track software development issues and manage testing. They troubleshoot and fix bugs, add new features, and enhance existing ones for better usability and performance. They also work closely with users, gathering feedback to improve the systems. Their goal is to ensure that the software development process is smooth, efficient, and free from avoidable errors.