Is FIRNS a Wordle Word?

Yes. FIRNS is a valid Wordle word.

It is not on the Wordle answer list, so it is unlikely to be a daily answer.

Plurals ending in S are almost never chosen as Wordle answers.

What does FIRNS mean?

Plural of firn: 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 snowcone slush is; can be hard or somewhat soft depending on recent and current weather conditions.

Definition from Wiktionary, available under the CC BY-SA license.

Starter word rating: WeakIn the top 43% of all valid Wordle words for testing common letters.
Vowels: I
Unique letters: 5 of 5
Common letters: I, R

Letter ratings use how often each letter appears in Wordle answers.

Tap its tiles to the colours Wordle showed, and the Wordle Solver lists every word that still fits, with the best next guess.