When you stroll around in Ranch Area, Snow Fireflies will randomly appear. Jump on it and the event will be activated and will appear in your Adventure Journal as an item. Snow Fireflies has a tendency to appear when Tomba jumps of a ledge to a lower level, which makes the collecting annoying sometimes. They are not affected if the curse if lifted and can also be found in Donglin Forest.
After you have collected nineteen Snow Fireflies, you should head to Donglin Forest and find the Snow Firefly researcher in the upper left corner of the area. To reach that area you must first clear to Switch with Donglin event to flip the platforms upside down. Equipping the Flying Squirrel Suit makes it easier but is not mandatory (you could also grapple your way) to make your way up. When you are there, talk to the researcher to trigger a cutscene. The event is cleared during the cutscene.