With EPUBReader you can read ePub files just in Firefox. No additional software needed!
Here is a little more about EPUBReader from the developer:
If you click on a link to an ePub file, you are normally prompted with the Firefox save dialog.
With EPUBReader installed, you don’t see the dialog. EPUBReader downloads the file, makes some processing and displays the ePub file ready to read! If you have got already downloaded ePub-files, you can read them by using the Firefox “File/File open” dialog.
Want to learn more? You can pick up EPUBReader from the Firefox Add-ons website.