Why doesn't Firefox show the correct default select option?

I had a similar problem, but after adding autocomplete="off" HTML attribute to every select tag it worked. [I was using Firefox 8]

Firefox preserves your selected form elements when you refresh. It's intentional. Ctrl+F5 is a "hard" refresh, which disables this behavior.

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Or Command+Shift+R if you are on a Mac

An easy way to prevent Firefox from caching the last selected option is to remove all of the option elements on page unload. For example (assuming jQuery):

$(window).unload(function() {
  $('select option').remove();
});