Firearms in the Attack on Titans is refering to flintlock muskets and pistols. They have been used by the military police since before the Titans came. These weapons are inaccurate and take 20-30 seconds to reload and must be reloaded after every shot. They are easily capable of killing human targets at close range.
They are ineffective against Titans in nearly every possible manner except for blinding them (temporarily of course). Although this tactic is not often seen in the Anime it was used when the 104th Cadet Corps cleared the Cadet Corps Head Quarters from lesser Titans in order to refuel their 3D maneuvering devices.