Your Xenforo JavaScript object is failing to work properly. Specifically, $.browser is undefined. Have you made any recent changes to your install? More specifically, have you removed the jQuery Migrate plugin, since that adds back $.browser support?
Try re-uploading /js/xenforo.js. The migrate plugin is the first line (first few characters should be "jQuery.migrateMute"). If that doesn't work, what changes have you recently made?
Try re-uploading the .js file I mentioned in the last one. I'm looking and comparing yours to the one that ships with XenForo and they aren't the same.
Unfortunately, I FTP files. Just gave a quick glance and the file you have recently reuploaded is still missing the first line (the migrate plugin). Not sure how or why.