A FileZilla developer works on the open-source software, FileZilla, which is a cross-platform FTP application. The developer's tasks include writing, debugging, and updating the codebase that supports the software's functionality, such as file transfers, directory synchronization, and remote file editing. They also work on ensuring the software's compatibility with various operating systems, enhancing its features, and fixing any bugs or issues. Additionally, they collaborate with other team members to improve the software's user interface and performance, and may also interact with the user community to address their needs and feedback. The goal is to maintain and improve FileZilla's reliability and efficiency.