Sure, Google may already have put out the Android M Developer Preview, but Amazon is only now getting round to basing its Fire OS on the previous release.
Today the mega-retailer announced a Developer Preview of its own, for Fire OS 5 that is. This iteration of Amazon's OS based on Android will be using the Lollipop codebase.
If you're a developer you can try it right now, but if you're just a consumer rocking an Amazon device you'll need to wait until "later this year" when Fire OS 5 will be officially released.
The fact that Amazon is only now using Android Lollipop as a base for its forked version of the OS shouldn't be surprising, considering that the update to Fire OS which used KitKat code only went out to devices at the beginning of May. So it's pretty clear that by the time Fire OS 5 (based on Lollipop) will actually be out, Google will have released Android M already.
The Fire OS 5 Developer Preview can be installed on a Fire HD 6 or Fire HD 7 tablet. If you don't have one to test on, Amazon is offering you a $50 discount on the Fire HD 7. You do however need to be a registered developer to take advantage of this deal. The whole procedure is described in detail at the Source link below, which is also where you can find the link for the Fire OS 5 download.