When using the Insert Stuff framework to add a YouTube video/other embedded media in a Content frame, the Firefox internet browser blocks the content because it is unsecure without informing the user that there is actually an invisible block around where the media should be. This makes it appear as if the insert failed even though the browser is actually blocking the unsecure content.
To resolve this issue, the URL that is linked from within the Content frames must use https:// and NOT an http:// URL. If the destination site doesn't have an https:// URL, then you might reconsider linking to this content. If it is necessary, ensure that your students know that they need to enable the ability to view unsecure content.