ASP.NET MVC - Firefox still keep selected value of dropdownlist after pressing F5

[watkinslahore]

I put an dropdownlist inside an ajax form, when change SelectedIndex the form will load new html. Then I press F5, the dropdownlist still keep the SelectedIndex even though the SelectedIndex in the returned html is 0.This only happened on Firefox (I ran on FF 15.0.1)Is there any trick to resolve this?