Circuit boards are epoxied, temp-rated wiring/insulation would melt not burn. There’s nothing in avionics to burn.
In the absence of proof of a suicide bomber blowing himself/herself up in the bathroom, I still think it originated in the septic/holding tank - an explosive that reacted with or wouldn’t be affected by water, (heavy bathroom use before final approach) either placed in the tank beforehand or inserted via the toilet or galley pipes.
Wiring can indeed burn—see Swissair for one example.
Wire insulation is plastic, it burns. #1 source of house fires in America is a short that that starts burning the insulation. Once your insulation is on fire it can travel anywhere in the structure.
Anything in there with a lithium battery?
A flare in the toilet. Someone in the front end crew would have access to a flare. it is Egyptian Air we are talking about. The front end crew was rather likely to be Moslem. One probably decided it was time to do his bit for Allah.