Appearance issues – large white spaces

I’m showing some weird spacing issues on both Firefox and Chrome on my blog, which is using Grace Under Pressure. I’ve noticed in the past that the more posts on the front page, my right menu sidebar runs out and just shows white.

Today however, when going to update my blog, I noticed there is a huge white bar between my side image and my posts in Firefox. I checked it against Chrome and initially, it didn’t show, but as I scrolled back up, not only did this white bar suddenly appear, but now there’s a huge white space at the top of my sidebar menu. I have a screen shot available, though not sure how to share it here, but it shows the oddness.

What exactly is happening?