If one wanted to connect a firewire peripheral to a 2011 MacBook Air, one could buy Apple's Thunderbolt monitor, connect the monitor to the Air and the peripheral to the monitor. That's kind of overkill, however, for simply connecting a Firewire music keyboard. Does any other method currently exist?
Nothing shipping immediately, however Sonnet did announce a Thunderbolt ExpressCard adapter that you could pair with a Firewire ExpressCard to give yourself some Firewire capabilities. Their site claims it will start to ship by December 14, 2011. At one point a direct Thunderbolt to Firewire adapter was announced, but I can't find any concrete info on it, so I wouldn't count on it appearing any time soon.
Overall Thunderbolt support from anyone other than Apple has been pretty slow to take off. There have been some suggestions that Intel was slow releasing final specs for manufacturers and/or getting production hardware out the door. I've also read that some manufacturers may be waiting for the 2nd generation of cheaper Thunderbolt chips to ship sometime in 2012. Lots of promise in the technology, and I think it will take off sooner or later, but it's still in the early stages unfortunately.
Apple announced a Thunderbolt to FireWire 800 cable at WWDC 2012. It seems that this cable will be shipping in July 2012. (The adapter is mentioned at the bottom of this page.)