Integrated widget is not working in FireFox


I have integrated “Visitor countries (world map)” widget into my site using iframe. It is working fine on Chrome and Safari, but not in FireFox. In FireFox I get this message: “Displaying Graphs in Piwik requires Flash”, but if I use the direct link instead of putting it in iframe it works on FireFox no problem, in other words Flash is installed on the browser for sure.

I would really appreciate any suggestions.