I have had the same problem for a couple of weeks with the drop-downs on Firefox on 3.6.13 (MS Vista) when trying to search for flights on the website. If I use Google Chrome 9.0.597.98 it works fine like Firefox use to. I even tried Firefox in safe mode to see if it was an add-on, still no drop-downs or calenders.
I'm a software engineer so I can give you any technical info you need.
Chris W