I think the default is to allow third party cookies in all browsers - it's an option to turn it off, which is meant to basically keep ad sites from tracking you as they display ads.
If third party cookies are not allowed you're out of luck if you're presenting information from another domain in a frame. Your only option is to ask customers to set an exception for your site if they have this option set.
The other option is - don't use cookies, but pass whatever information you need via query string 'license plates' rather than cookies (which is a lot of work but possible).
+++ Rick ---
from Geneva, Switzerland