Tree inside Panel -> strange behavior in Firefox

Hi, I have a SplitPanel (horizontal) and in the left part of the SplitPanel, i have a Panel.
Inside that Panel i want to show a Tree. Works fine in other browsers, but in different versions of firefox, the tree is always drawn outside the browser window (i don’t see it). Sometimes when is use the bar inside the SplitPanel to shift the panels, a part of the tree becomes visible and when i click on of the items, the tree jumps to the correct place inside the SplitPanel.

The Panel that contains the Tree is also not drawn (i can see the border) correctly (shifted slightly to the left and top) and also jumps to the correct place inside the SplitPanel when i am able to click one of the tree-items.

Anyone who had the same problem?

When i replace the Panel with a VerticalLayout, everything is ok, but then i lose the border which i really want…

Any suggestions?