Firefox users can download Epubreader
, a free add-on that lets you read the contents of any ePub book right inside their favorite browser without requiring any special software.
ePub is basically a zip file that contains a collection of text files and images. This add-on will simply uncompress that zip file and display the contents inside your browser.
In regular course, if you click on a link that points to an ePub file, Firefox will prompt you to to save that file onto the hard-drive but with this add-on installed, the ebook will directly open inside your Firefox tab like any other web page.
Additionally, you can open any ePub file thatís on your desktop using the File -> Open File menu in Firefox. All the ebook are automatically stored in your Firefox profile under a separate sub-directory.
Also see here http://techie-buzz.com/helpdesk/how-to-open-epub-files-epub-ebooks-reader.html