There is a method to figuring these kinds of issues out.
What you do is first give everything a basic flat grey material using the material override feature in the scenes tab.
Do a test render and see if you are still seeing the firefly noise. If you are seeing the noise then you know it is related to the lights in your scene,
If you don't see the noise, start adding objects to the excluded list of your material override, with test renders in between.
Keep doing this until the fireflies show up again. then you know it is related to the object/material of the object you just excluded.
Once you figure out what specific thing in your scene is causing the issue it is easier for others to help figure out the problem.
With that being said...
It is most likely related to your fire material light being too bright. To get around this i would lower the brightness of the fire and then if you want more "fire" ambient light add an additional invisible sphere light to your scene.
