There's quite a few HTML validation errors which could be related. Most browsers are much more robust in their handling so it could be related, or it could be some quirk in Opera.
It might be worth enabling the XenForo default style and seeing if the issue can be reproduced there. If this has been deleted, you can re-create it by creating a new style with no parent. Then get the user to try with that style.
After that, I'd also recommend disabling the user flags add-on (as the flag images seem to be randomly appearing) and also widget framework (temporarily) as this is where I spotted some odd markup (<td> tags being used in invalid places).