You haven't passed it all the parameters. The WeaponImage::onFire method doesn't use %client in its parameters, it uses %this,%obj,%slot where %this refers to the weapon image, %obj is the client's player and %slot is the mount slot or something.
You'll need to do:
parent::OnFire(%this,%obj,slot)
in the parenting bit, and use
function WeaponImage::onFire(%this,%obj,%slot)
for the method.
So all together:
package fireprevent{
function WeaponImage::onFire(%this,%obj,%slot)
{
if($fireprevent == 1) // Off
{
messageClient(%client,"","\c6 Sorry, Firing has been \c0DISABLED.");
return;
}
else
{
parent::onFire(%this,%obj,%slot);
}
}
};
activatepackage(fireprevent);