Zero Apps currently on the Android Marketplace can be installed on the Kindle Fire. Zip. Zilch. Nada. All Apps for the Kindle Fire will be installed through the Amazon Appstore. And because Amazon maintains control over the Amazon Appstore, Amazon will be in a position to guarantee that everything submitted to the Amazon Appstore will be compatible with every device capable of tapping into the Amazon Appstore.
For the purposes of App distribution, the degree of "fragmentation" between Kindle Fire and all other Android-based devices, is on the same order of magnitude as the degree of "fragmentation" between Windows Phone 7 and hp webOS. It's a nonsensical correlation because the two ecosystems, by intentional design, don't overlap.
If a 3rd party developer wants to target their Apps to the Android ecosystem, then they will submit them to Google's Android marketplace.
Subsequently, if they also want to target their apps to the iOS ecosystem, then they will redevelop their apps to be compatible with iOS, and to satisfy Apple's editorial guidelines, and and submit them to Apple for approval.
Finally, if they also want to reach the Kindle Fire ecosystem, then they will have to ensure that they meet Amazon's editorial requirements, and submit them separately to the Amazon Appstore -- it just so happens that the process of porting the app from generic Android to Amazon Fire may be relatively painless due to largely identical toolchains and APIs.