The box does NOT show in Firefox if you are not logged into Facebook, tested using Private Window No addons. There is no place to log in or anything, its just not there. It will only show if you are already logged into Facebook prior.
The box DOES show in IE, both as a logged in Facebook user and using In-Private browsing as not logged in.
Thanks for writing in! I have tested your site, but I can see the Facebook comment box with the option to login (https://snag.gy/lRatod). Could you please try testing it again by clearing your cache.
That’s definitely odd, I see the markup on your page, but there’s no output in Firefox logged out of Facebook as you say, there are no console errors or script errors either.
Can you provide some FTP details and WP admin details in a private reply please, so I can take a closer look at this?