firefox margins

when i have a wide screen monitor, it causes mozilla to have excess space on the side of the browser. obviously because its wide screen, and i was wondering if there was a way to stretch out the page. making the width of the page wider without increasing font size or increasing picture size.

some websites, like AF, stretch out the the page, so it fills up the whole browser, while others like yahoo! do not, and leave a big block of white nothing on the sides.