Html header looks different in IE and Firefox

I am designing this site for a friend: https://mrpkbb.com/blog When I view the blog in Internet Explorer, the green bar of the html menu sretches beyond the blog’s border—In firefox–it doesn’t reach the border. I would sove this problem just by adjusting the border’s lenght on the original image–but that would only solve the problem for one browser–I am stumped as to fix the problem so that the green line of the image is aligned with the green border perfectly in both broswers.