Alignment on Facebook - Firefox Browser

This issue has been raised before but the CSS fix doesn't seem to work for me on Firefox at least. The problem is the popup generated off the Facebook button disappears off the bottom of the screen and it's impossible to scroll or drag it into view.

I added
#wdsm-popup_wrapper { top: 10% !important; }

via My Custom CSS but it doesn't seem to resolve it in the Firefox browser.

My page is viewable at:
http://bookbzz.com/sign-book-deals-new-release-emails-get-free-book/

Any help gratefully received...

TIA
Lucy