I just tested it in Safari as well, with both gmail.com and my iGoogle page, both of which force the 330M to come on. It almost looks like the 330M kicks in when the chat pain in either gmail or iGoogle finishes loading. Surely discrete graphics are not necessary in this instance.
I have a lot of add-ons in Firefox, Safari is completely untouched. I never use it. I wonder if one of the "Labs" features in gmail could be causing it?
EDIT -
I just tried another gmail account that doesnt have any Labs features enabled and it still uses the 330M in both Firefox and Safari.
I had this problem until I uninstalled the google voice and video chat safari plugin. After restarting the browser it now stays on the Intel all the time. Maybe that will work for you?