Microsoft has released a fairly extensive study of browser use on laptop computers and the effects on battery life. Not surprisingly, IE 9 comes out in the lead overall. As far as Mac browsers go, Firefox 4 takes the crown, followed by Chrome and Safari. Opera brings up the rear in the study.
I’m not sure about the usefulness of the information, as your use of the browser is only one aspect in terms of how long your MacBook’s battery lasts. And how many people use enough Watts of power just surfing the web? But the information provided is interesting nonetheless.