Header Size Issue (Firefox only)

I’ve checked my new header in Firefox, Opera, Safari and IE. Firefox makes the whole blog (and so the header image) bigger–which is good because the transblack theme uses a rather small header. But bad because Firefox is blowing up the image and making the text and art a bit fuzzy.

Any suggested fix appreciated!

NOTE: This theme comes with a background header image. I then created my own header image and inserted it into the stock header image using PS CS4. If there’s a way to provide and call a different, larger header image version in Firefox (or make the whole blog bigger in all browsers), I’m for it.

Thanks in advance!