Thank you for the detailed explanation. We're able to reproduce the issue but unfortunately it seems that the problem is in the way Firefox displays IFrames - when you use the RadHtmlPlaceholder in windowless mode we place it inside an IFrame that is over the Silverlight plug-in.
In the attached project you can see that if you directly use an IFrame to display the html5tutorial page the issue will present in Firefox (it's fine in IE and Chrome).
Unfortunately at the moment there's not much we can do but if you have further questions feel free to ask.
p.s. After some research we found out that if you add "allowfullscreen" to the IFrame it starts working correctly if Firefox:
You can do this in code behind with something like this:
var iFrame =
iFrame = iFrame.Insert(7,
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