As nouns the difference between fern and firn

is that fern is any of a group of some twenty thousand species of vascular plants classified in the division Pteridophyta that lack seeds and reproduce by shedding spores to initiate an alternation of generations while firn is a type of old snow which has gone through multiple thaw and refreeze cycles and thus is made of numerous small icy grains, though it is not nearly as saturated with water as snow-cone slush is; can be hard or somewhat soft depending on recent and current weather conditions.

As a proper noun Fern

is {{given name|female}} from the fern plant.