Is this an updated org from CRM 2011? If so, there could be a problem with the JScript on the form or even an issue with the ribbon on the form.
We had a generic error message box that rendered on the main page and in forms after moving to rollup 11 on CRM 2011. We used tracing in Fire Fox and found that there was a problem with a reference in the ribbon. We think this was something done
by a ribbon editor from codeplex because it was found on 2 of our orgs. The reference in the ribbon had a duplicated file extension.
Hope this helps.
Jason Peterson