It works for me in a normal Firefox browser window. Please note that if you use the "Private Mode" browsing feature, Firefox will automatically block Javascript and sites that can 'track' you. Facebook definitely falls into this category, which means that Firefox will automatically block some features like the Like Box.
There's nothing we can do about this as that's Firefox's behavior and the user chooses to browse the web in Private Mode.
The only recommendation I can give is to use our Channels feature which allows you to pull in a Facebook Page feed and display it natively on the site. However, to do that, you need to be an administrator of the Facebook Page and I'm guessing that's not the case with the Page you are trying to display.
I hope that at least helps explain the issue, but if you need anything else, just let me know.
Thanks,
Alex