We're using Firefox in our daily work as well as many other users of Telerik Reporting, but we've not noticed or been reported such an issue. In fact you can go ahead and test whether you would get the same behavior in our online demo reports here
We believe that there is something else messing up the report viewer's toolbar e.g. global css or themes for your page. Please exclude all those and verify if the problem is gone. If it persists, please send us your .aspx page with the viewer along with your web.config, and we would be able to provide more info once we review them.
the Telerik team