What is the firefighter's rule and what is the effect of it?

"In its most classic form, the firefighter's rule involves the question whether a person who negligently has started a fire is liable for an injury sustained by a firefighter who is summoned to fight the fire; the rule provides that the person who started the fire is not liable under such circumstances. [Citation.] Although a number of theories have been cited to support this conclusion, the most persuasive explanation is that the party who negligently started the fire had no legal duty to protect the firefighter from the very danger that the firefighter is employed to confront. [Citations.] Because the defendant in such a case owes no duty to protect the firefighter from such risks, the firefighter has no cause of action even if the risk created by the fire was so great that a trier of fact could find it was unreasonable for the firefighter to choose to encounter the risk." (Knight v. Jewett, (1992) 3 Cal.4th 296, 309-310, fn. 5.)