Mozilla Firefox is a free and open-source web browser developed by the Mozilla Foundation. It is the second most used browser worldwide with over 450 million users. Firefox allows users to customize their browsing experience through extensions, themes, and built-in developer tools. It supports many web standards and passes Acid2 and Acid3 tests, providing strong security features like SSL/TLS encryption and anti-phishing protection. Firefox is available in many languages and locations worldwide, making it one of the most widely accessible browsers.