Sorry the lack of vocabulary, my mother tongue is French.
As Firmin was over 34 years old at the beginning of the conflict, he has not been incorporated in a regiment which was at first line.
He was in the 37e régiment territorial and his barracks was in Auxerre.
http://fr.wikipedia.org/wiki/R%C3%A9giment_d%27infanterie_te...http://fr.wikipedia.org/wiki/37e_r%C3%A9giment_d%27infanteri...It seems that there is no rank on the picture (?).
I imagine his role was the one of a "gendarme" during the WW1 as said
The "livret matricule" of people before 1900 classe (=b. before 1880) are not on line at the Yonne web site.
Gendarmerie is still part of army in France.