Image border display issue - Genesis/Exchange/Firefox

Hello WPMU people -- Any chance somebody can help me figure this out? I've looked everywhere and am totally stumped.

On this page:
http://weteachwp.com/product/wp101-weekly/
...only in Firefox, only at the 2 largest screen sizes, the big featured image is missing one of its borders, as if 1 pixel is being cut off.

The site uses a Genesis child theme and iThemes Exchange. (I know, I should be using WPMU stuff!)

Any suggestions much appreciated! Thank you very much in advance.

Best,
P@tty