I finally figured out the problem.
I looked for things in my sidebar that might be causing problems with its size. The only thing that looked possibly too long was the Feedblitz email subscription box. I tried setting a smaller limit for this box, but it didn't seem to make a difference. When I removed it altogether, I was able to go back to pretty much my original widths.
So for whatever reason, Firefox seems able to resize the email box to the sidebar with no problem while IE seems to resize the sidebar to fit the email box.
There is still a button in the sidebar for anyone wishing to subscribe by email, but the white box will be gone for now until I can figure out how to get it there without resizing the sidebar.
Whew. That's 2 hours of my life gone....................