A 'Mech that has fallen or dropped to the ground may fire some of its weapons as long as neither of its arms has been destroyed. The warrior uses one arm to support the 'Mech as it fires, and so the weapons on that arm cannot fire. The warrior may fire all weapons mounted on the other arm, and the 'Mech may fire any weapons mounted in its head or torso (including rear-mounted weapons). A prone 'Mech may not fire any leg-mounted weapons.
Add a +2 to-hit modifier for firing when down, in addition to any other standard modifiers. For example, if a 'Mech runs and then drops prone, it receives a +4 to-hit modifier for any attacks (+2 modifier for running + 2 for firing when prone = 4), in addition to any other modifiers that might apply.
A prone four-legged 'Mech (provided it has 4 legs, and has taken no hip critical hits) fires as though it were standing - the +2 to-modifier does not apply.