Okay, before anyone gets upset, I read through all the previous sidebar posts, and none seem to be the same issue I have. Most of those posts don't have solutions anyway.
Our site worked great for over a year (no change in theme) until yesterday. My daughter posted 2 posts, and the sidebar slipped in IE but not in Firefox. When we deleted the posts, things went back to normal. I retyped the posts from scratch, and the stupid sidebar slipped again. You can type just about anything as a new post, and the sidebar slips.
I looked at the validation page, and we have a bunch of errors, but all are old. Therefore, I'm thinking that none could be responsible for this new change.
The only difference is that I think my daughter used Firefox on her new Mac to post, but I can't imagine that this would matter.
Any assistance would be GREATLY appreciated, as we can't post anything new on the site right now.