I have a similar problem on the homepage but it is intermittent. however I notice both on your demo of this theme and my version, the one-page checkout brakes in Firefox. If you select checkout as guest and continue, you will notice that a new attribute is assigned to the ID main column of 750px. this only happens in Firefox from what I can tell, but it prevents a user from completing the checkout process.
Anyone any thoughts as I cannot find where this attribute is even coming from and why it is just for Firefox.