I heard a few decades ago already that human fire watchers look specifically for smoke during the day (as the saying goes: where there's fire, there's smoke) - and I suppose indeed for fire glow at night. Both are easy to pick out for human observers.
For automated fire detection I'd think of IR, especially at night when the sun doesn't mess up things: fire is much hotter than the surroundings - and will produce a column of hot air, both of which should be very clear on an IR image.
During the day it's harder - smoke detection should work based on overall brightness of an image. Smoke will be much less bright than the surrounding clear sky, even if it's a cloudy sky. Clouds emit light (due to the sun shining on them from the top), smoke not so much.