Firefox bottom margin problem

Check out my blog: https://www.onanimation.com

I just finished the theme, and I’m happy with everything but a gap I can’t figure out at the bottom of the page. It should stick to the browser window like it does at the top. I’ve checked out my tags and they’re all closed properly. I can’t figure out why. I read something about Firefox handling things differently and this is a common problem, but no solution. How do I fix this. It looks fine in IE and Chrome.

Thanks, much appreciated. ??