A ModernizrJS developer utilizes the Modernizr JavaScript library to detect and features in user's web browsers. This tool aids in ensuring that websites function properly across different browsers by identifying which features are supported. The developer uses this information to create conditional code for fallbacks or enhancements, optimizing user experience. Furthermore, they may contribute to the Modernizr open-source project, helping to develop and refine its capabilities. They work closely with front-end developers, sharing insights about browser capabilities and limitations, and assist in implementing progressive enhancement strategies for web design.