Miki,
Sorry for the delay in getting back to this. We've now tested with Firefox with 2 different computers, a Mac and a Windows 7 PC. Additionally, we tested IE on the Windows computer. In all cases, when we click the blue login with Facebook button, the authentication popup from Facebook that asks for email address and password both appear correctly and in front of your website.
Have you tested with other computers? Can you let me know:
* Is it the Facebook authentication popup that is going behind your window, or is it some other popup that you aren't seeing?
* If you move your browser window, can you see the popup correctly?
* Can you try turning off any add-ons or extensions you have enabled for Firefox and IE and try again?
We've simply never heard of this issue from anyone else, and we can't recreate. My best guess is that it's an add on to your browser (or a malware protection software) that's trying to hide those popups.
Thanks,
Alex