I'm working on a site where data has to be fetched from a 3rd party soap feed so we use ajax to give the user some immediate feedback - eg. searching animation. (searching flights fwiw). I've just added a feature where the available departure airports are context-sensitive (ie, they change when the selected destination is changed). It works great except when the search is running. ie. if you change your destination in the search form before the results have been returned from the search request, the request is queued and isn't run until after the results have come back.
I've tried in Safari, Chrome, IE9 and FF15 and firefox is the only one where the departure airports can be changed while the search is running.