Hi Timo,

not sure what makes it do that... I'm betting the table in the filemanager will need a div or something added to it in order for Firefox to recognise when the treemenu has expanded below the visible window.

However, to get you sorted in the meantime, you could add this to your stylesheet:

.FileManager
{
 height:400px;
}

Make it enough pixels to cover what you need. Although, if you need like heaps and heps of pace down the page then this might not be sufficient either. In that case, try adding the issue to this place here: http://support.dotnetnuke.us/

Good luck,
Rob