No report that I've seen has given *any* reason for his fhimng. Given his rather extensive criminal past I suspect it may be more complicated than you seem to want it to be.
Since you beat this dead horse multiple times just for shits and giggles and if you bothered to read the news reports, it was claimed he was trying to RECRUIT for ISLAM at WORK. Do you honestly think that is going to go over well in a Baptist Bible Belt state? Probably not. It don't take Einstein to put those two things together as a probable cause for getting fired then, does it?
Not to mention, if he got fired for being insubordinate because of his Islamic ranting and raving at work, he probably mistook that as an affront to his phony faith. So given that possible context, and based on his fakebook posts, he would in fact be acting out a crazy Jihadist fantasy which was directly inspired by Islamic extremists like ISIL. But even still, that makes him being a terrorist a tough sell if he is acting alone and he's really just a crazy, criminal asshole.
And regarding his criminal past, they had to have done a background check before hiring him in a company of that size. Some companies even try to hire excons for certain jobs because they often tend to be more reliable and harder workers since they are grateful for anyone hiring them at all. While I'm not discounting his criminal past as a reason for being fired, it just don't sound very likely.