The FireingForce determines how much force is applied to the shell to resist gravity. The actual speed of the shell is determined by Velocity, and the effect of gravity can further be modified by the GravityModifier. It determines the arc of the projectile with relation to the muzzle velocity. A Sherman, for instance, has a firing force of 110, which "lobs" the shells, as opposed to a Panzer IV's firing force of 70, which has a much more level shot, given that the muzzle velocities are equal (100).
Values range from 10 for the Yamato and 30 for the Katyusha to 110 for the Sherman.
Source and more details
The MDT is incorrect, this property determines the amount of physical recoil on the parent object. It is commonly seen on tanks, causing them to rapidly roll away from the direction that they are firing.