There was a known issue with scrollbars and FF5 that has been fixed after the Q2 2011 release. Please try upgrading to the latest internal build and see if this helps. If it does I would advise that you wait for the official Service Pack to use in the actual web site, as the internal builds are not always suitable for production servers. You can, of course use it anyway in the live site, but it is not fully tested and some issues may still arise.
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