Does downgrading Lion to Snow Leopard also downgrade firmware? Ever since I installed Lion I've had problems with wifi. Connections would drop, and once wifi was turned off it wouldn't turn back on. Now, I can't use it at all - it says "No hardware installed."

The same problem also exists in Windows on my Boot Camp partition, so I think it's a firmware or hardware issue. I'd like to fix it by downgrading to Snow Leopard, but would this also downgrade the wifi firmware?
No. Firmware is outside the scope of traditional OS updates (e.g., 10.7.2). On Macs, there is actually no way to revert a firmware update (even if you download a prior revision).