A new technology has been developed by the folks at Firefox.com that allows users to dress up their favourite browser (Firefox of course) with a more decorative feel than just changing the theme and buttons. It's call Firefox Personas.
Firefox Personas allows images to be displayed behind the buttons of the browser while not being too intrusive.
It's freely available to all users and comes all varieties of designs. Also, the Firefox labs people have allowed you design your own "persona" and publish it to their gallery for others to download.
I highly recommend you give it a go yourself.