This is a known bug, as documented here LINK
Great, so I've set about adding the Firefox-specific workaround, as seen in this demo. LINK
//initiate the original postback again
Which works fine with normal RadButtons. But, I've found if the RadButton has a <ContentTemplate> declared, the postback is not executed, and the confirm keeps calling the OnClientClicking method which opens the RadConfirm again.
I've copied the code from this Demo into my own test page and reproduced the issue, it is very simple to reproduce. Simply add a ContentTemplate to the second button.
And now the confirmation cannot cause a postback in Firefox. We are using Telerik version 2016.2.607.45