Lucidor is a an EPUB reader for Linux that is based on Firefox. I have found it to the best among the EPUB reader options available on Linux.
The features I like about Lucidor:
Table of contents displayed on the left of content. This is the #1 reason I had to stop using FBReader, because it could not show the TOC beside the content.
Auto-size option to scale content to fit window size.
View different books in tabs. I wish every ebook reader did this!
A personal bookcase to which your books are added.
Tried with: Lucidor 0.9.10, Firefox 37.0.2 and Ubuntu 14.04