Amazon's phone, tablets, and other Fire devices run Android, but it doesn't come in the form you see on most devices. They all run a variant called Fire OS. The current version is based on Android 4.4, though this doesn't matter all that much once you take into account the sheer volume of changes Amazon makes.
Nevertheless, Fire OS 5 is on its way, and it will run Android Lollipop. Today Amazon has made a developer preview available for download. This will let software creators see how their apps perform on a Fire HD 6 or 7 running the upcoming version and spot any issues ahead of the official launch later this year.
Amazon has also upgraded the App Testing Service on its Developer Portal homepage to tell developers if their apps are compatible with Fire OS 5 without having to go through the effort of acquiring and flashing a new build to their device. But if you do want to acquire a tablet to test with, Amazon is offering $50 off up to two Fire HD 7 tablets. Hit up the link below for the details.