I believe you are looking for mac-address command feature. You can either use mac-address auto command or define the mac addresses manually if there is more than one pair of cisco ASA firewalls in your environment since there are chances that the automatically generated mac addresses might coincide.
FYI, mac-address auto is a global command whereas if you wish to assign the mac address manually, it has to be done in interface config in interface config.
Please find the info below:
http://www.cisco.com/c/en/us/td/docs/security/asa/asa-command-reference/I-R/cmdref2/m1.html
HTH
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AJ