Well, as for EFNet being unique in some way to cause mIRC to crash... I would analize all your scripts where the network you're connected to causes anything different to happen. Try using /debug to pinpoint if the crash is relative to any server event, and perhaps turn /remote off to see if it still happens.

The only way I know how to crash mIRC is with $regex. Perhaps that may help.

Well. At least I won lunch.
Good philosophy, see good in bad, I like!