Well, I'm happy to report this issue seems to be resolved. I didn't do anything. It's even possible I haven't restarted this PC in the last two weeks since my last comment. I often have many, many windows open over days on-end doing my job. I like to sit down at the PC and pick up where I left off and not have to locate and re-open a couple dozen programs and windows, so I sleep the PC at night during larger jobs and restart when I have cleaned up the desktop at the end of a project. I DID restart when this issue began, as this has always been the first tech "go-to" with troubleshooting.

My guess is there has been a Firefox update since and I just didn't pick up on it. It always happens when I'm busy and just want the flipping browser to open for me, so it ticks me off, I click away and get back to what I was busy with. There is always a page loading delay the first time opening Firefox after an update. Always at exactly the wrong time for me!

So, my advice for anyone who experiences this problem is to just let it ride. Two weeks after I reported this problem, suddenly Outlook Live with Firefox was working normally, allowing me to mark as read and delete emails once again. In the meantime I could easily go to Edge or Chrome to delete and mark as read.

The problem resolved itself.