The actress and stand-up comic shared multiple clips to her Instagram Story detailing her eventful evening Thursday night ... beginning when she says a friend told her people in her neighborhood caught a guy trying to set fires on a back hill -- saying she thinks they caught the guy in Woodland Hills.

Cummings seems to be referring to a viral incident that's circulating on social media ... featuring residents holding down a man with a blowtorch before calling the police -- though it's unclear if this individual actually did anything Whitney's claiming.

Law enforcement sources tell us they're investigating the incident as a "possible arson."

Fast-forward a bit, and it looks like Whitney's decided to take matters into her own hands ... leaving the house with her beau ... explaining they're going on patrol to stop alleged arsonists -- and, they've got weapons.

Well, they've got objects that look like weapons at least ... 'cause Whitney holds up a pair of wooden swords -- and jokes they're ready to duel anybody looking to light up the region.

Whitney says they can't really kill any suspected firebugs ... so, they gotta bring enough crazy to the situation to scare them off -- hence the swords.

For how serious the L.A. fires are -- they've already burned more than 35K acres and killed at least 10 people -- these clips certainly provide some levity ... with lots of laughter and jokes. It's a sharp shift from earlier this week when Whitney bawled on video while being evacuated.

Multiple celebrities have claimed arsonists are responsible for all the fires in L.A. ... though right now, only one person has been connected and accused of starting the Kenneth Fire -- the other fires are not thought to be the product of arson at this time.