Since you didn’t specify a specific model of Cisco PIX, I can only speak in generalities. I don’t think these devices support MAC filtering. At least I can’t find any evidence of it. The following document provides an overview of PIX and makes no mention of it.
Frankly, even if your firewall supported MAC filtering, it may not matter. MAC addresses can easily be spoofed via software (e.g., under Windows, you merely have to use Device Manager and reconfigure the driver, not very difficult). I suppose if you're attempting to block an iPhone or some other network appliance where you typically don't have that kind of access (e.g., XBOX, PS3, internet radio), it might be more effective. But in general, trying to control access based on MAC or IP address is a losing battle and easily defeated by a determined user. Not unless you supplement MAC filtering w/ some sort of “lock down” mechanism on the client.