I don't think this does what you want, it reduces fire strength at (-1, -1), (-1, 1), (1, -1) and (1, 1) (the diagonals)
Probably what you want is every adjacent square, the easiest way to do that is to iterate over every square in a radius of 1 of the origin, and just treat the origin specially.  Also I'm seeing field updates freezing, not sure why.