Hi bobdowns Open Developer Edition, type about:preferences in the URL and press enter. Then check the box that says: Allow Firefox Developer Edition and Firefox to run at the same time. Then Restart

Like I said, I've checked the box that says to allow both to run.

Sorry for that.

By default, Developer Edition is meant to use a separate profile so that it can be run side by side with FF. were you using it side by side successfully before, and then it changed after updating?

I have had both running concurrently for months with no problem. Only in the past few days has the non-developer edition failed to quit, repeatedly, regardless of whether the developer edition is also running.

I've even restarted my MBP and then run the non-developer edition alone; and, then tried to quit it, to no avail. Only a Force Quit will make it quit.