This is a Firefox 3 security feature for the <input type="file"> HTML control. There is nothing wrong with our component.
Some quotes from the web about this Firefox 3 feature:
"I really don't think this is a good idea, to disable direct text input in the <input type="file"> input box, as a security "fix" against a theoretical attack."
"The interface is confusing as there's a text input box, but when you click the input box, a file open dialog pops up, that goes against any user expectation of a text input box."
"Another problem with this "feature": because you can not edit in the text box, it is IMPOSSIBLE to remove a file once it's in there."
"It is a completely pointless security measure and moreover, it greatly affects usability."
I can't reproduce this, using Firefox 2 or IE 6.