Bizarre Scroll-Bar Pops Up When Clicking Header

I’m working on my first venture into custom WordPress theme creation, and I’ve been surprised at how easy it is, all things considered. However, I’ve run into my first little bump in the road.

My header graphic (for lack of a better term; it’s actually on the sidebar in this design) is giving me some trouble. Whenever I click on it to return to the homepage, a bizarre, horizontal scroll-bar pops up. When the page loads, however, it goes back to normal.

Also, the problem seems to be specific to Firefox (I’m running 3.5.3). It’s not happening in Chrome (4.0.223.11). I am unable to test IE, at the present time.

And, if it matters, I’m running a local install of WordPress, with zero plug-ins or modifications beyond the creation of this theme.

Here’s a low quality picture of what my design looks like normally. Aaaaaand, here’s another picture of what it looks like after I click on the header image.

EDIT – Actually, the equally bizzare border around the clicked image probably shouldn’t be there either. =|