There are 2 main properties of fire: warmth, light.
At a microscopic scale, you can get the warmth directly by respiration. By respiration, I mean processing food (glucose) with oxygen to produce energy. In fact all living organisms have a fire burning in them. Only very very slowly. In fact SE has a very quick check on that too: https://physics.stackexchange.com/questions/116960/how-much-energy-in-form-of-heat-does-a-human-body-emit which states that an average adult male dissipates about 120 watts per hour. Which is nearly equivalent to burning 4 kgs of coal per hour.
So then. In order to get warmth at microscopic scale equivalent to fire, you oxidise high-energy chemicals into oxides and derive energy from them. If you want an even higher energy compound than glucose, try respiring with ethyne (acetylene) and oxygen instead of glucose and oxygen. Trust me, you (as a microscopic organism) would NOT regret this!
For light, you are going to use bioluminescence as a living organism. Vertebrates (fish) do that. Marine invertebrates do that. Even bacteria and fungi do that! Why can't you, an intelligent microscopic life form, can do likewise? And if your bodies are unfit for that, you can always domesticate some bioluminescent bacteria or grow a few molds of that fungus. Then harvest it and carefully seal the shiny part (probably some compound of phosphorus) in a non reactant microscopic bag (probably some membrane or something) and carry it wherever you want.