I believe you're experiencing this: https://confluence.atlassian.com/display/CONFKB/Widget+Connector+not+Displaying+Embedded+Content+when+Using+Firefox+or+Chrome
Could you try this workaround and let us know if it works?
Try to use another browser to view the video clip or clean all cookies to see if that helps. Otherwise, you may update your FireFox to the latest version, but if the problem persists, try to download the YouTube video clip directly with a free tool and view it offline without using browser.
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