Recently I made a tree with hair particles instead of leaves. At first all worked fine, but after a few renders a lot of fireflies appeared. But when i preview render in graphic window at same amount of samples, everything is OK.
The final render and viewport render aren’t always exactly the same. You can reduce fireflies by heading to the
Render (properties tab) > Sampling > Clamp Indirect
Put in a value like 8 to begin with and slowly decrease it to the highest value with an acceptable visibility of fireflies.
It could be a matter of resolution: fireflies pixels get mixed with good ones if preview is a smaller window than render
i second this. my go to render resolution is 3072 x 2048, im always surprised what shows up 
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personally, i clamp indirect at 1. in all the cases ive tested, theres a small to no visual set back with a huge noise improvement. unless you specifically are looking for caustics.
beware it’s not only caustics! Clamping indirect to 1 also clamps reflections of bright lights