EPUBReader is the best EPUB reader/library for Firefox. It's an add-on for the Firefox browser which can read EPUB books from anywhere. EPUBReader will cease to work in a few months due to major changes coming to Firefox, and rewriting this very complex add-on will take time.
Please consider pledging $10euros, about $10usd, to keep the project alive. Donate here. You use your credit card to make a donation. Your credit card is not charged unless the project meets its goal. When the goal is met, only then is your credit chard charged.