I see now, that the problem is related not to Firefox, but to Facebook account.
While JFBConnect works fine for my main account, it displays this blank window for my second "test" account. I can give you the login email and password for this Facebook account if this can help fix the problem.
I'm seeing the white, blank, box as well when trying to login. Can you go to the
Facebook Developer Area
, go into your app and then the "Status & Review" section. In there, make sure the large setting at the top for:
Do you want to make this app and all its live features available to the general public?
Beyond that, I'm not sure why you'd see the white box, but if it persists, let me know and we'll help how we can.