I've tried Tools.Options but it's not obvious (to me anyhow) how to define a trusted web site.
Firefox does not use the same Trusted Site system that Internet Explorer uses. With Firefox you can specify general privacy and security settings and then add exceptions for certain sites.
You can manage privacy permissions for individual sites via the page info window. You can manage the defaults and also add/remove sites to exception lists via the Firefox options.
In Firefox 6 and newer, you can also manage everything via the new aboutermissions page if you wish to give it a try.