I'm getting the OS X crash reporter instead of Firefox's.
I can reproduce this at will with the crashme addon.
I'm a few days behind (2013-08-15), but I get our crash reporter dialog on my Nightly on OS X 10.8 with the same steps. What Nightly version and what OS version are you on?
Closing this as I happened to come across it while searching for another issue. I can't reproduce it, it's pretty outdated, and no answer from the original reporter.