I think your best bet is to use
Mozilla Prism to make a site-specific browser for any sensitive sites.
I use them for all webmail, shopping and banking sites I use regularly, and if I want to buy something somewhere else, I start another browser with just that site (e.g. I normally use WebKit, but open Chrome to shop etc.).
You can make SSBs with Safari, too, but I don't recommend it, as all Safari instances use the same cookie store. To my mind, that defeats the purpose of an SSB.