Just reading up on @doctorlinguist's TenFourFoxBox single-site browser app for PPC Macs. This aspect sounds nicely secure.
"You may be dealing with sites that you want to be completely separated from your main browser so that there is no chance information can leak from one to the other. Even though TenFourFox runs them both, the foxbox is in “its own private hell,” so to speak, and unless you say otherwise private data in a foxbox is always cleared when you quit."