Starting Issue After Sitting For A While...

Is there an aftermarket alarm? Any DTC codes at all? Any idle issues?

Backfire is a sign of fuel in the exhaust. There is a Ford TSB about leaking/stuck open fuel injectors. One of the symptoms is a crank with no start and maybe back fire.

Consider an injector cleaning/flow test service such as InjectorRX.com. Once cleaned and tested they are as good as new. Cheaper than new.

If the cam sensor has been incorrectly installed I could see this causing backfire.

Does it make any difference if the pedal is held half way open during cranking (do not pump)? If the MAF is disconnected?

Do you have an ODB2 scanner? If so, would be nice to find out what the PCM "thinks" the fuel pressure is and the MAF air flow.

I had this same problem. It went on for about 3 months before my check engine light came on. The code was for the cam position sensor. After some research I decided to change out the alternator first. ( this can screw with the system and throw a cam code)
The problem went away.

Mine is starting to do the same thing and sometimes it will cut back going down the road or cut off when pulling up to a stop light with no codes.

I noticed the same thing with mine, usually I try to start it every two days. If there is white smoke in the exhaust it could be water in the gas ( just run some HEET on your next fill up). Another point to consider is spark plug replacement. I usually change mine every 3,000. that ended the heasitations during pulls.