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I've cloned your JotForm to check the issue with checkbox. I tried to edit the cloned JotForm with Mozilla Firefox v33 and Safari 6. But, I was not able to see any issue with the widget.
This could be an issue with the browser cache. We request you to try clearing the browser cache by following the steps provided in the guide http://www.wikihow.com/Clear-Your-Browser%27s-Cache
Please let us know if the issue still persists. We will be happy to look into this issue further.