You might want to download a copy of the source, and pass that through Dreamweaver's validation feature. That can sometimes turn up a missing end tag. WHY a tag would be missing is hard to tell, but it could be a plugin or something else.
I passed the URL through the W3 validator, and the first two errors/warnings would be considered "fatal"
The W3C Markup Validation Service
It only works in Firefox and other Webkit-based browsers because those browsers are correcting for the error.
Others like
id=hidden
rather than id="hidden" (which it should be)
could cause problems depending on the browser.
Anyway, some of this can be corrected by going through the theme and making the necessary corrections but other things may be due to WordPress messing up (
SEO pack, for example).