If the FedEx tractor-trailer jumped the median as some have reported there are numerous things that could have caused flames, both before and after. If it was a diesel engine, flames are less likely but not impossible from a fuel line or tank rupture. Brakes? I’ve seen those catch fire on trucks headed down a long steep incline. Flammable material inside the cab igniting? Something in the cargo? Sounds as if the road was fairly well traveled. There will be other witnesses.
From Redding in the north to Bakersfield in the south, I-5 is pretty dang flat terrain. Could probably rule out a brake fire.