Only Apple retail stores or Apple Authorized Service Providers can unlock these computers protected by a firmware password.
If you cannot remember the firmware password for your MacBook Air, schedule a service appointment with either an Apple Retail Store or an Apple Authorized Service Provider. If you plan to visit an Apple Retail store please make a reservation at the Genius Bar using http://www.apple.com/retail/geniusbar/ (available in some countries only).
What do such engineers do to recover or reset the firmware password? I long ago forgot my MacBook Air's firmware password, but haven't needed it until now. I'm quite comfortable opening up my laptop, even if it voids my warranty, but cannot find any information on how the reset is performed.
With older models, one needed only to change the hardware configuration (e.g. amount of installed RAM) and then reset PRAM three times to force a reset of the firmware password. I presume this is not possible because the MacBook Air has no user-removable parts (since RAM is soldered to the motherboard, for example) but it is not clear what alternatives exist.
Apple probably require all those in the know to keep it to themselves, but no doubt some hacker has figured it out. I'd be grateful if they would share!