The Collaborative International Dictionary of English v.0.48:

Firman \Fir"man\ (? or ?), n.; pl. Firmansor. [Pers.
   ferm[=a]n.]
   In Turkey and some other Oriental countries, a decree or
   mandate issued by the sovereign; a royal order or grant; --
   generally given for special objects, as to a traveler to
   insure him protection and assistance. [Written also
   firmaun.]
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

FIRMAN. A passport g ranted by the Great Mogul, to captains of foreign
vessels, to trade within the territories over which he has jurisdiction; a
permit.