The NFE7YT is a theory created by Furby fans, which is a result by analyzing Furby releases.
Since the 1998 Furby has released, Furbies came out every 7 years:
Bold: Upcoming Furby.
This means that since the last Furby has been released, a rumored Furby will release in 2019.
This hasn't been proved because Hasbro hasn't announced a 2019 Furby yet, but the 2012 Furby also got announced late.