If you have a game developed with an older version of Unity (such as Unity 4.5 or 4.6, when ‘UnityPlayerNativeActivity’ was the default activity), then on Fire OS 6 your game might have a 1-2 second blank white screen before the game loads for the very first time into the device memory. The issue is due to Android 7 (Android N - Nougat) cold start feature while the app loads to memory and waits for all the work to be completed. This issue happens for all Android 7 (Nougat) devices, and is not specific to Fire OS 6.
To fix the issue, upgrade to the latest version of Unity, such as Unity 2017 or newer. If upgrading is not an option, you can try updating your AndroidManifest.xml file by changing UnityPlayerNativeActivity to UnityPlayerActivity, which has been available since Unity 5.0b12. For more information, please see https://docs.unity3d.com/Manual/android-manifest.html