Anyway, is your fire switch soldered? You may have used too much heat on it if it is.
All your grounds at the 510? Any of them loose?
Did you test this outside the box before you installed it?
I don't see anything wrong with the diagram , although I would rather use a fuse than a p-fet
The only things that should be causing auto-fire may be: bad soldering at key locations, or, exposed areas contacting the box shorting it out, or over-heating the chip/parts while soldering.