FirefoxConfiguration
Turning on Scaling and Printing from Print Preview
Open a new tab and type "about:config" then search for "print.whileInPrintPreview" and set it to true, you should see buttons for scale, landscape and portrait when you next try to do a preview. Some of these features don't seem to function properly, but enough do to make this change worth it.
I would also recommend install the Print/Print Preview extension.