Another Fedora Mystery
I ran into a strange Fedora quirk. I started Firefox and every single web site I tried to access through my bookmarks failed to load. Eventually I found the root cause. Somehow
browser.tabs.remote.autostart had been enabled. I had no reason to do this on my own. I have no idea how this option got toggled.
As I use four different distros on two different systems and none of the other environments had this happen, I can only guess what happened in Fedora.