"A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete.
I can click on "Stop script" or "Continue" button. I click on continue and it's working fine. This issue most probably caused by the work load for the script like loading folder and etc. It's fine if it's not consistently happen.
To know more what happen, please read on http://kb.mozillazine.org/Script_busy_or_stopped_responding