The different Date/Time Pickers contain of an input field and a button that opens a calendar view in a popup. Usually, the background color of the button changes when it is clicked. In Firefox, nothing happens, because it seems to block the :active pseudo class. This DOJO demonstrates the behavior.
In other, similar input widgets like the NumericTextBox, this problem does not occur, because the .k-selected class is set on the button instead. Still, Firefox blocks the :active pseudo class.