To call someone “fit to burn” is to be chastising someone in a
relatively extreme way. For example, when someone has done something really bad to another person.
Usually used when describing someone else’s behaviour - you wouldn’t
necessarily use this about someone you yourself are chastising.
Also sometimes used when the alleged culprit hasn’t actually done what they’re being accused of, but
the person who is doing the accusing has been told that they did.
Possibly quite specific to North-West England.