I am not quite sure what the problem is here. There is a firefox epub reader
here though I have not tried it. ADE works on my Linux box though I tend not to use it, the standalone Calibre reader is my reader of choice though it does not come as a separate install and may take to many resources for some people to install it. I have recently started using Sigil and it makes a fair reader with a low resource overhead if you can ignore the editing tools at the top of the screen.