I had the same issue yesterday, though it could have been because of my developer profile of Firebug. I didn't investigate any further yet.
Please let us know, if the issue reappears for you.
Please note, that we currently have a problem with the Script panel claiming there's no JavaScript on the page. This issue seems to be a Firefox problem and is already reported at
https://bugzilla.mozilla.org/show_bug.cgi?id=689101.
Sebastian