[ Thanks to Dmitri
Popov for this link. ]
“You can use a dedicated application like FBReader or
Calibre to read and manage ebooks on your machine, or you can do it
without leaving the convenience of the Firefox browser courtesy of
the EPUBReader extension.“With this extension, you can open any ebook in the ePub format
directly in the browser using the File -> Open File command.
EPUBReader conveniently splits the main window into two panes. The
left pane contains the hyperlinked table of contents, while the
right pane displays the book.”