What do you mean by "it shut down Firefox"? Did it warn you that it was going to close the application? Because if it didn't, that's most likely a crash (even if the crash reporter dialog didn't show up afterwards for whatever reason).
What do you mean by "it shut down Firefox"? Did it warn you that it was going to close the application? Because if it didn't, that's most likely a crash (even if the crash reporter dialog didn't show up afterwards for whatever reason).