This issue is probably related to an extension that you have installed on Firefox that's accidentally keeping your computer from entering sleep mode.

Try running Firefox in Safe Mode and see if the problem goes away. If the issue is not present in Safe Mode, the problem is likely due to an extension.

Hope this helps.

I'll have to play with that. It's difficult to determine, as *sometimes* it works just fine. (Like right after I received this suggestion!)

Much like when you bring your car into the mechanic's, it will always work fine when you are trying to figure on the issue :)

The issue isn't related to any specific website, is it? Some elements (like videos) can prevent Firefox from entering hibernation mode.

Exactly that. As mentioned in the original post, I can not tie it to any particular tab/website. And I do not believe I closed any when it decided to work the other day. Still need to observe its behavior, I guess.

My firefox default install and a few addons even when left running my Window 10x64 1803 will still go to sleep/hibernation mode. And without any browsers open it goes to sleep/hibernation quicker. So I suspect something loaded into your Browser is telling the O/S to stay on rather then let it's power mode work correctly. Disable all addons from running close Browser and open again and see if that doesn't change. If it does then you got Addons/Extensions problems. As I have only 4 Addons and my system will go to hibernation regardless.