EPUBReader - Read ePub just in Firefox
http://www.epubread.com/en
"EPUBReader is a Firefox addon which let you read ePub-files just in the browser. You don't need to install additional software!
If you click on a link to an ePub-file, you are normally prompted by the Firefox "save as" dialog. With EPUBReader installed, the ePub-file is downloaded, processed and directly displayed ready to read!"