Render hangs FF13 beta Mac

Using Cutout Madness, using Save Image As (rendering) hangs on blank or regular images above a certain size on the Mac with FF13 Studio beta. To reproduce:

Make sure that Tools / Options / Interface / Reduce preview size for larger images is CHECKED

1 - Run Cutout Madness
2 - Source / Blank Image
3 - Set to 4000 x 3000 with blue color, then hit OK
4 - Hit Save Image As
5 - Use PNG any channel format, hit OK
When rendering gets to the end, it just hangs, but you can hit Cancel. Using the top command in Terminal shows FF is not using much CPU, like it is waiting for something. Let me know if this can be reproduced.

Workaround: UNCHECK Tools / Options / Interface / Reduce preview size for larger images, then let preview completely draw the large image, then do a Save Image As