As nouns the difference between firn and pirn

is that 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 while pirn is a cone shaped spool or bobbin on which thread or yarn is wound used most often for weaving.