If you click on a link to an ePub file, you are normally prompted with the Firefox save dialog.

With EPUBReader installed, you don't see the dialog. EPUBReader downloads the file, makes some processing and displays the ePub file ready to read! If you have got already downloaded ePub-files, you can read them by using the Firefox File/File open dialog.

If you experience any problems, please let me know, I'm very happy to help you. Please contact me via the link of my support-website.