You can do this with a Persona, such as the Firefox B persona. It will replace the toolbar background with an image containing the Firefox logo - http://www.getpersonas.com/en-US/persona/16

Thanks Tony, I want to keep the FF4 default theme and colour and just add the Firefox logo on top. Is there a way to click and drag the logo from a site onto the existing theme? Thanks.

No easy way to do that, you would have to go into the Firefox application files and modify the default theme.

That's a little too complex for me. Thanks anyway.

oops. You can see how adept I am at these things. I was trying to put one of those images in my picture.