I was only able to hypothesise the cause, as that happened to correlate well in my minuscule sample size; your data point appears to have invalidated that. I re

I was only able to hypothesise the cause, as that happened to correlate well in my minuscule sample size; your data point appears to have invalidated that. I regret not being able to isolate the cause properly. I would still run the user account experiment — your described symptom matches exactly what I experienced. It should only take a few minutes to make an account, log into it and run Firefox. What I do know: * The problem started after a major macOS version upgrade, almost certainly Sierra. * The problem occurred on my desktop and my laptop, but not my wife's computers that saw largely similar install history but more plain-vanilla usage patterns. * The contents of the Firefox profile directory do not matter; the problem occurs with a completely fresh profile. * The contents of the ~/Library directory do not seem to matter; creating a new user account and moving the entire contents of ~/Library back fixed it. * The problem could not be replicated within any other user accounts on the same machine where the problem occurred. * I was unable to create a faulty user account.