The Kobo app is also in Amazon Appstore. However it will not side-load DRM-ed ePub.
Overdrive Media Console can probably be side-loaded using .apk at Overdrive's web site (if it doesn't show up in Amazon Appstore). Mantano does not yet deal with library DRM so if you want to read a library book in ePub format on Fire OMC is an option (as is Aldiko, which will also do Adobe DRM PDF). OMC of course also does audiobooks and music. The OMC ePub reader itself is very basic, I would not use it unless it is the only option.