I think that the 5.3.3.1 on Wikipedia is a typo. According to
Amazon's Fire & Kindle Software Updates page, the latest version available for 4th and 5th generation tablets is 5.3.1.0 and the new 6th generation Fire HD 8 runs an incremental update: 5.3.1.1.
Amazon has promised Fire OS 5.4 with Alexa by the end of the year.