EDIT: Just got a cert from the new service. Here's a screenshot of the cert chain. Amazon is issuing them: https://imgur.com/VGos0eY
Amazon had applied to be a Root Certificate Authority to Mozilla and Android since June 2015 [1]. And it's been in pending for public discussion recently [2], which is one step away from being inclusion by Mozilla[3](aka becoming a root CA).
Once they are vetted and being included in Mozilla's Root CA program, they will be accepted by Firefox browser and on Linux. And after vetted in Android, it will be accepted by Google Chrome/Chromium browsers.[4]
[0] http://i.imgur.com/s2Uijes.png
[1] http://www.geekwire.com/2015/amazon-wants-to-be-your-ssl-cer...
[2] https://mozillacaprogram.secure.force.com/CA/PendingCACertif...
[3] https://wiki.mozilla.org/CA:How_to_apply#Public_discussion
[4] https://www.chromium.org/Home/chromium-security/root-ca-poli...