Stephen contributed to the mozilla/gecko-dev repository by addressing a session management issue that caused duplicate tab groups after session restore. He improved the reliability of session restore by ensuring that automatically restored tab groups were removed from the saved session state, preventing duplication and maintaining a cleaner user experience. Using JavaScript and Python, Stephen updated the session restore logic and expanded the test suite to simulate scenarios such as closing the last window, validating correct behavior under real-world conditions. His work demonstrated a strong focus on bug fixing, thorough testing, and careful handling of session data to reduce user confusion.