Library e-books are one approach. Until Amazon announced (and then launched) content through OverDrive, people who wanted library books on their ereader often asked for those library books to be ePub. The thinking has been that once we've all digested the launch of kindle format on OverDrive, the demand for ePub would be reduced. But it hasn't been eliminated, for a variety of reasons, including one my friend N. poignantly brings to my attention: libraries in other countries.
Will Amazon support ePub on the Kindle Fire (or any other Kindle)? Probably not. And even if they did choose to support ePub on the Kindle Fire natively, it might or might not satisfy the goals of the people who want it (that is, if you've got an ePub formatted book with DRM and it isn't Amazon's DRM, why would Amazon support that DRM? And why would the owners of that DRM help/let them do so?).
The Amazon AppStore already exists. When it rolled out, there was a lot of speculation regarding what, precisely, Amazon had in mind for this AppStore (viz. make money off everyone else's tablets or roll out a tablet of its own? We know the answer now, obviously). But it seems wildly unlikely that Amazon would run an AppStore and then not let you buy the Apps in that Appstore for use on its own device. It's possible. Just wildly unlikely. So one answer to the ePub/kFire question is, are there Apps in the Appstore that can deal with ePub? The answer is Yes (Aldiko and and Moon+ are on the top of the list); notably, there's a _Kobo_ App available (for free!) in the Amazon Appstore (altho I have not yet found a Nook ereader).
Finally, Amazon's policy on the Kindle Fire with respect to rooting seems to be: we won't help you, but we won't work very hard to stop you, either; the expectation is that people will do this more or less from day one.
So if, like N., you are wondering whether you will be able to read ePub library books on the kFire, the answer is a qualified "Yes". If you are prepared to root the kFire, almost certainly. Probably even if you _are not_ prepared to root the kFire. And if you'd like to do some experimentation in advance, you could run an experiment in advance by borrowing an android tablet or smartphone from someone, buying the Aldiko or Moon+ apps _from the Amazon AppStore_, getting one of the books onto the device and attempting to read it through Aldiko and/or Moon+. If it works, you can be reasonably certain it will also work on the kFire.
Will people who want Amazon to support ePub "natively" be made happy by this solution? NO.