Firefox opens normally and runs properly -- until I close all its windows. When zero windows are present, and I return to Firefox and ask for a new window, Firefox hangs. To resolve the problem I must Force Quit the app, which generates a crash report to Apple. Occasionally Firefox has crashed on its own and generated a crash report to Mozilla, but this is rare. I have tried disabling all add-ons, but that had no effect.
As long as there is at least one Firefox window open, the app functions normally. This problem only occurs when going from zero windows to one window.
Disk permissions and plist file did not help. Resetting Firefox did clear the problem, although with the collateral damage of having to set things up again. Better to be working than not, but I don't know what caused the problem or whether it will come back.