Pages behave differently in different browsers

In Firefox and Chrome, everything works perfectly fine as it always did, but ever since the latest update (3.0.3), I’ve had potential commenters tell me that they wanted to comment, but no form appeared! After testing it, it turns out that this happens very frequently using Safari, Internet Explorer, and Opera, all up-to-date.

Not only do comments not appear, the entire right sidebar is missing as well. I am completely baffled by this, because at the very least all the “modern” browsers should work the same (all of the above except for IE), right? I switched themes last weekend from Pilcrow, and that seemed to fix the problem, but now I am using Magazine Basic and the problem is back! It only seems to happen on the individual post pages, but I can’t think of how to fix it, or even what the problem is in the first place. I’d rather not wipe my install and restore from a backup before this started happening, but I’m starting to think I have to.

Anyone have a clue what’s going on and how to stop it from happening?