Force Postback on Arrow Navigation on DropDown

Hello everyone,
I am having a cross cross-Browser problem. I have a web application (ASP.NET) with a drop down box. When it has focus I can navigate through the items in both firefox and IE using the arrow keys. However, in firefox I have to press enter in order for the selection changed event to fire. I would like to have firefox behave like IE without having to press the enter button.
Thanks in advance