This seems to be caused by a bug in FireFox. The DateInput tries to select the nearest segment when you click in it. Τhe click event, however, is wrongly emitted when you select a range and then release the mouse button outside of the boundaries of the input. As per the spec, this should not happen - no click event should be emitted. Thus, we see this difference in behavior. Here's an example with a plain input.
However, reviewing this, I have to admit that the selection mechanism of segments is not very intuitive at times. There's actually a logged issue on the topic from a while ago. We'll look to give this higher priority and revise the selection mechanism if possible. At first sight it looks illogical to completely move the user selection when there is one, regardless of whether there's a click or not.
Just subscribe to the issue, if you want to get notified when we've enhanced this behavior.
Let me know if I could provide further information on the topic.
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