Thanks for your last reply. We opened a support ticket yesterday and tried the latest build and it still doesn't work. Here is the URL to a development version of the current project we are working on where you can see the problem.
If you view it on Mac OS 10.3 or higher using Firefox 2.0 or higher, you will see the PanelBar flicker before it starts the animation.
I have built our project against the source code and have narrowed the problem down a little, but haven't been able to fix it.
In RadPanelItem.js in the "_displayChildren" function, when the following line is set and the object is set to visible, it forces the panel bar to open briefly before it start the animation. This causes it to flicker in Firefox.
aniamtionContainer.style.height = "auto";
If the height is not set to "auto" the PanelBar doesn't animate smoothly, but it doesn't have the flicker problem.