Fortigate 'E' models

[ZeroInterrupt]

No 5.2.x firmware for any 'E' models. Looks like they are forcing you to use the 5.4.x.  Kind of unfortunate as the 5.4.x firmware would not be my first choice on a production unit.   

[MrSinners]

The "E" models bring new architecture and hardware, the 5.2 software would have to be fully updated to incorporate that, which most likely isn't worth the time and resources, unfortunately. 5.4 is not my first choice either currently.

[Paul_Dean]

The best thing so far about 5.4 is that it makes managing on a mobile device easy. Try it.