There is already a previous link at the bottom of multi-page threads that will take you back to the previous page. But I really don't see what is wrong with using the browser's back button. That is, after all, why they put it in there for.
if you don't want to use the back button you could alway use the right mouse butten if you useing windows i can' rember if it is thier on linux and i have know idea about mac.. maby they are using full screen mode on IE. :lol: and you can add it using the templets if you want that on your forum.