Fire OS

Amazon Fire OS is a mobile operating system based on Android and created by Amazon for its Fire tablets, Echo smart speakers and Fire TV devices. It is forked from the open source Android operating system and includes proprietary software, a customized user interface primarily centered on content consumption, and heavy ties to content available from Amazon's own storefronts and services.
While the Fire tablet line has always used customized distributions of Android, Amazon only began referring to the distribution as Fire OS beginning with its third iteration of Fire tablets. Unlike previous Fire models, whose operating system is listed as being "based on" Android, the "Fire OS 3.0" operating system is listed as being "compatible with" Android.
Features.
Fire OS uses a customized user interface designed to prominently promote content available through Amazon services, such as Amazon Appstore, Prime Video, Amazon Music & Audible, and Kindle Store. Its home screen features a carousel of recently accessed content and apps, with a "favorites shelf" of pinned apps directly below it. Sections are provided for different types of content, such as apps, games, music, audiobooks, and video among others. A search function allows users to search through their local content library or Amazon's stores. Similarly to Android, sliding from the top of the screen exposes quick settings and notifications. Fire OS also provides integration with Goodreads, Facebook, and Twitter. X-Ray is also integrated into its playback functions, allowing users to access supplemental information on what they are currently viewing. The OS features a user system, along with "Kindle FreeTime", a suite of parental controls which allow parents to set time limits for using certain types of content.
Fire OS 5, which is based on Android 5.0 "Lollipop", uses an updated interface. The home screen now features a traditional application grid and pages for content types as opposed to the previous carousel interface. It also introduces "On Deck", a function which automatically moves content out of offline storage to maintain storage space for new content, the speed reading tool "Word Runner", and screen color filters. Parental controls were enhanced with a new web browser for FreeTime mode featuring a curated selection of content appropriate for children, as well as "Activity Center" for monitoring usage by children. Fire OS 5 removes support for device encryption; an Amazon spokesperson stated that encryption was an enterprise-oriented feature that was underused. In March 2016, after the removal was publicized and criticized in the wake of the FBI–Apple encryption dispute, Amazon announced that it would be restoring the feature in a future patch.
Amazon's ecosystem.
Fire OS devices comes with Amazon's software and content ecosystems such as Here WeGo with a clone of Google Maps API 1.0. Amazon cannot use the Android trademarks to market the devices.
Fire OS devices does not have Google mobile services, including the Google Play Store or proprietary APIs, such as Google Maps or Google Cloud Messaging. Google Play Store can be installed, and third-party apps can still be sideloaded via APK files, although full compatibility is not guaranteed if the app depends on Google services. 
Members of the Open Handset Alliance (which include the majority of Android OEMs) are contractually forbidden to produce Android devices based on forks of the OS, therefore Kindle Fire tablets are manufactured by Quanta Computer, which is not an OHA member.
List of Fire OS versions.
The releases are categorized by major Fire OS versions based upon a certain Android codebase first and then sorted chronologically.