Hi, I do have the same issue with Firefox 21.0. I discover the new api update so I follow the guideline to update to the new widget (a pity one with no way to customize the display, but it’s an other problem). The widget works on Google Chrome (can’t try on IE, because I am using Ubuntu).
I also update my google analytics script to be sure and remove a viveo video script who was in conflic with others js of the page.
My firefox has no special restriction and no blooking plugins. There is no display in firebug console window, so I have no way to found the source of the problem. Here is my blog address http://www.cedric-augustin.eu/ (the widget is inclued on the top right of the home page.
Hope this will help you to found the problem.