Right sidebar appearing below the content area

I’m having a peculiar problem tonight. A little while ago, I was tinkering with a form in the Visual Form Builder. When I refreshed the page on which the form appeared, the right sidebar suddenly started appearing below the content area. I’ve had this happen once or twice before, usually because there’s an unclosed div tag in a post or page, but I couldn’t figure out why it happened this time. I tried tinkering with the form settings, thinking I had a code nesting problem, but it didn’t help. Neither did unpublishing the page on which the form appeared or turning off the VFB plugin.

Finally, I decided to restore a backup (database, plugins, themes, uploads, everything but the WP core files and root) from before I started tinkering, figuring that would resolve the problem. It didn’t, even though all the changes I made in the form were discarded when I restored the database.

I tried switching to a different theme (Twenty Twelve), which put the sidebar back where it was supposed to be; switching back to my Frontier child theme put the sidebar back under the content area.

Thinking that the theme files had somehow gotten corrupted, I tried switching from the child theme back to Frontier parent, but the same thing persisted. I then tried deleting the parent theme and reinstalling a fresh version from WP — no change. Temporarily disabling all of my plugins didn’t make any difference either.

The problem does not appear in Chrome, Opera, or IE 8, only in Firefox. It’s not just me; I asked someone in another city to try loading the site in Firefox and they got the same thing.

I am baffled and out of ideas. If you have any suggestions, I’d be most grateful.

This is the site:

here