The following is a known issue for the Composite Page Designer:
CR 6624231
A user-generated portlet for a project found in the tutorial operates when deployed on portal server in the application server web container (using a Solaris 10 x86 zone), but it does not render properly in Firefox.
To work around this issue, change the portlet to thin rather than thick. This will not fix the problem entirely but will make it easier to view.