ePub Viewer is a Firefox extension designed to facilitate the reading of ePub and OPF files directly in the browser. By adding a context menu option, users can easily open these files without the need for additional software installation. The extension features a 'Reader View' interface that includes three customizable color schemes: light, dark, and sepia. Additionally, users can adjust font size and family settings from a convenient left-side panel, enhancing the reading experience. Full-screen mode is also available for those who prefer an immersive reading environment.

This extension enhances usability by displaying a Table of Contents, allowing users to navigate chapters effortlessly. It also remembers the current page number and book URL, enabling users to continue reading from where they left off when opened in a new tab. Recent updates include the ability to load local files and a loading percentage display, further improving the overall functionality of the ePub Viewer.