Hello Stafan Müller,
By the time Q3 2008 was released FF 3.5 didn't exist. This is why there are problems with that version of FireFox. I suggest that you upgrade to the latest version of the Rad Controls suit. You will find this issue as well as some other annoying problems fixed.
However, if upgrade is not applicable, you can use the workaround that we have prepared. You can find sample as an attachment.
the Telerik team