We were unable to reproduce the problem you described using Firefox and Safari on a Mac running OS X v10.5 Leopard. There shouldn't be anything about a MacBook Air specifically that would make any difference.
However, the cart will not work without the buyer having a live Internet connection, so if that buyer loaded your page and then lost his WiFi signal -- maybe it was a tenuous signal to begin with, or he carried the MacBook somewhere out of range -- then the cart would fail to load for him until he regained connectivity. There's also a remote possibility he has a broken cart-tracking cookie which clearing cookies would solve, but this seems unlikely if both Firefox and Safari were affected.