Since I can't add them to the original post I include here a screen shot of the most recent warning window I encountered. I'd copy and paste the contents instead if your warning boxes allowed text copying. Hint, hint.

Please fix FF and release the fixed version soon so it becomes useful again. Don't tell me to use safe mode because that is not the environment in which the failure occurs.

Let's deal with those unresponsive script messages.

Type about:support in the URL bar and hit Enter.

Look at the list of Extensions and try to match up an extension with the code in that error message - the two lines containing Jetpack. Then try disabling those extensions.

I found a match up. The jid0-dII4p... matched the "Short Link" plugin. I disabled that and will wait a bit before I decide it was successful and the problem is solved but frankly I'd be quite surprised if I see that one again.

I believe I saw problems with other scripts prior to that one and will see if they yield to this diagnostic.

I really thank you for cluing me to about:support and this way of diagnosing unresponsive script problems.

It's been ten hours of browsing without a recurrence of the script hang so I consider this problem happily solved! Regained performance loss I saw with 31.0 as well. Thanks again.