This is the 9892nd most frequent French word.
fir
The French word "fir" does not exist; it may be a misspelling. If you meant "sapin," it translates to "fir tree" in English.
Here, 'sapin' is used as a noun to refer to the tree itself.
Le sapin est un symbole de Noël.
The fir tree is a symbol of Christmas.
In this sentence, 'sapin' describes the object that was decorated.
Nous avons décoré le sapin hier soir.
We decorated the fir tree last night.
The word 'sapins' is in plural form and indicates multiple instances of fir trees in a descriptive context.