If you just upgraded to Firefox 40 and get blank popups, Ghostery may be to blame. I was getting blank popups anytime a page opened a share or save dialog (tried Twitter, Pinboard and Facebook).
Here’s what I was seeing when trying to share anything:
![I Can't Has Popups! :(](/wp-content/uploads/2015/08/Mozilla_Firefox_and_Enabling_Developer_Mode_on_Mac-540x364.png#center)
After some troubleshooting, I disabled Ghostery and everything works like a charm now!
![Share_a_link_on_Twitter_and_Enabling_Developer_Mode_on_Mac](/wp-content/uploads/2015/08/Share_a_link_on_Twitter_and_Enabling_Developer_Mode_on_Mac-540x361.png#center)
Of course, this means that you won’t have protection from tracking in between. I’d suggest using Disconnect till Ghostery is fixed.