Lucifox - the Free EPUB-reader addon for Firefox
@Book Club

Lucifox is a Firefox addon that lets you search online OPDS catalogs for ebooks and read EPUB formatted ebooks right in your browser. Lucifox is available under the GNU GPL v3 and is maintained by Swedish developer Ordbrand. Norwegian translation will soon be available.

https://addons.mozilla.org/en-US/firefox/addon/lucifox/