I'm sorry for the late reply, we are busy currently with the next release of RadControls for ASP.NET AJAX which should be rolling out next week.
I've tested your markup and there indeed is some flickering in Firefox. Looks like a very strange issue and it seems related to the animated gif rendering that we use for root item backgrounds and the background color of the body. If you remove the background color only, the issue will go away.
If this doesn't work for you and you are willing to sacrifice the animated gif background of the RadPanelBar, you can open the gif image in a image editor and save only the first frame (or whichever you like) as a png, then override it in you page with:
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