Drop-downs and listboxes (<select> controls) just display options for the user to select. They do not do math on their own, but they can be scripted in various ways. There might be differences in behavior between browsers that your web people didn't notice in their testing, so perhaps that led to a bug in the page. I don't think you were given enough information to make a judgment about exactly what was going wrong, so it's impossible to say whether some change in Firefox created the problem or whether it was a programming error from the beginning.