As AfroKen mentioned, fires at night cause inherently high-contrast situations--the fire is to use a technical term, a "jillion times brighter" than the night sky...so, what you need is a way to bridge the contrast gap,and the solution is FLASH!!!!
But, you need to gel the flash with an orange-ish gel in order to keep the flash's light output from looking too white. A 1/4 to 1/2 CTO orange-ish gel slipped onto the flash will allow you to use a small amount of flash on the people,and it will look "real", not faked. You cannot just blast the people with a full-power flash burst and not make it look a little bit faked, unless the fire is positively huge, like a bonfire. But,as long as the flash's output is warm and orange-y, you can get away with a lot, since the eye is good at spotting differences and incongruities,and orange light comes from....hot,glowing things like the sun at night, molten metal, hot filaments, and fire and coals...
When you use a gelled flash, you MUST take control of the white balance--many cameras default to FLASH white balance,so you need to make sure that an AUTO white balance does not mess things up. Setting the WB to a fixed pre-set is a good idea.