FireFox Addon capturing whole desktop when using "Capture visible part"

HS works great for a time, then it starts capturing the desktop instead of the browser client area. It's a new issue and only started a couple weeks back. But I haven't installed anything new programs.
 
The open image count in HS, other running apps, the time since last reboot, network/VPN connection states, free drive space, target save directory, etc., appear to have no effect on when it happens.
 
I'm curious if anyone else is seeing this and if you have any hints to resolve it. Also, are there any known add-on conflicts? Even if it's not an "official" observation and something you think might have caused it.
 
Edit:  It's just started happening again (20150611 - 1730 UTC):
You can see that the capture port is shifted down and to the right. The capture is the proper size, though.

Hi Greg,
 
I wouldn't spend a great deal of time on it. Mozilla may actually get around to fixing FF's hard issue bugs eventually, and your fix will be broken.