Firefox has a new add on where you can change the background/skin of the browser.
Go Here to download the app, once it is downloaded, restart Firefox. Then, in the lower left hand corner of the browser, there will be a little fox, click no that, and you can choose your Firebox skin
The print screen button is just besides the pause button and above the backspace. At least here, but I have a laptop, so might be different on regular keyboards.
