I suggest returning the Macbook Air and getting a Macbook with Firewire. USB interfaces will give you latency when monitoring the recording signal, unless you have one that allows for hardware monitoring. (Perhaps someone else can speak about this. My interface is supposed to have this, but I've never used the feature. I also don't know what will work with Protools, I use Logic.)
Getting a Firewire to USB adapter will almost certainly give you latency issues, and could also introduce compatibility issues. If you can live with a little latency and can't return the Macbook Air, I suggest a plain USB interface. It's a simpler option than using the existing one with an adapter.
You also have another option, if your budget allows: Get a dual USB/Firewire interface with hardware monitoring. That'll not only let you monitor the signal being recorded in real time, it'll let you use it with pretty much any computer. (But there will still be latency when using it with USB, even USB2.)