I just installed Fire Fox 3 (beta).The first thing I noticed was the change to the forward and back buttons.

All browser buttons are not created equal. The back button gets much more use than the forward button. From Nielsen:

The Back button is the lifeline of the Web user and the second-most used navigation feature (after following hypertext links). Users happily know that they can try anything on the Web and always be saved by a click or two on Back to return them to familiar territory.

Since the back button gets much more use, the designers at Mozilla have grown it in size in comparison to it’s brother the forward button. Allowing for a larger target makes it faster for the user to accomplish their task. (see Fitz Law)