In case of Firefox i think each browser instance shares the same cache. You may want to try with IE where each instance is different. Having said that..the portlet driver is a light weight portal meant for testing the portlets, if you are looking at the portal, you may want to try Project WebSynergy(
http://portal.dev.java.net ). Both the portlet driver and Project WebSynergy use the same Portlet Container.
Regards,
Deepak
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