APM red led goes solid!

Yesterday i upload AP2.78b to my apm.
When i connect apm 2.5 to MP , the red (arm / disarm) led goes solid and HUD shoes ARMED, i try to disarm it with my radio tx but not succeed.
I then disconnect and power apm separately , red led still goes solid. I have followed all the procedure in
APM: Plane to install it my custom built plane.
Any help how to resolve this issue would be greatly appreciated!!!

I have done the following things to solve the issue:

a) Upload fw to two diff apm’s.
b) Upload fw with diff MP’s.
c) Change the Radio TX
d) Upload from internet , upload a custom fw , upload from arduino IDE
e) Upload from a diff laptop
f) Upload fw on old apm2.
g) Upload diff AP fws.
h) Upload w diff data cable.
i) Googling leads me to a blog on diydrones “Solid Red LED A when switching on APM 2.5”.
I followed TCIII suggestion there and "Save my vehicle parameters and then using the Terminal Mode and CLI, do a reset, erase, and then reload the parameters and see if that helps correct the situation (but no good).
j) then clear the df logs coz there was a case w Pixhawk switching on w solid red led and remedy was to format the SD card.

But still the red led solid w AP fw.

Anyone suggestions about this issue or anyone who experiences the same problem.

Solved the issue just after reading this http://plane.ardupilot.com/wiki/arduplane-setup/first-time-apm-setup/arming-throttle/.

In MP, ARMING_REQUIRE parameter was set to 0 [ disabled].
Set it to 1 and the red is flashing now. And i can arm the apm now.

With that setting at 0 the plane would be armed as soon as you plugged in the power, hebce why mp was saying armed in the hud.

Curious as to why you have used ver 2.78 rather than a more recent version?