The role of Senior Fullstack Developer at Girona Softwareentwicklung GmbH involves modernizing the company's core product, transitioning from Legacy-PHP to Symfony and React. Key responsibilities include developing fullstack features, maintaining existing code, conducting code reviews, and mentoring apprentices. The ideal candidate should have around 4-5 years of experience in software development, with a strong grasp of PHP, Symfony, React, and relational databases.