If you change a dropdown and refresh the page, Firefox seems to ignore the selected attribute.
Just had the same issue, believe me it's been more than 10 hours struggling with this stupid firefox behavior, i have 7 dropdowns, each of them will trigger an event and fill in 24 hidden inputs, so you can imagine having the right option selected with 24 wrong input values!!! the solution i finally found is to reset the form with Javascript adding this line of code:
window.onload = function() { document.forms['MarkerForm'].reset(); };
PS: the inputs have the values pulled from a database, so resetting the form does not empty any value but in a way tells firefox to go back the hell to selected=selected option!