I wasn't able to reproduce the issue on our online demos. Could you please test some of the demos, to see if you experience the problem? For example, you could check the following: http://demos.telerik.com/aspnet-ajax/dock/examples/forbiddenzones/defaultcs.aspx
I suspect that this is CSS issue, so please look for global styles applied to <DIV/> elements. To help us reproduce the problem locally, please send us a sample code (that we can easily run locally), and the full CSS code applied to your page. It would be easier if you could provide a live url, where the issue can be observed.
PS. Could you please tell us the exact version of Firefox that you use?
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