If the problem does not exist when you disable all add-ons, then one (or maybe even more) must be causing it.

Have you tried disabling all add-ons and then re-enable them one by one till you find the culprit  ?

You won't have this add-on, cause that would be too obvious :

https://addons.mozilla.org/en-US/firefox/addon/keep-awake-screen-only/

Thanks, McCoy. As I tried to say in my posting I must manually disable "all Add-ons". To prove this, I disabled all Add-ons, then painstakingly enabled each one, one at a time, then tried to sleep. Each and every one prevents the system from sleeping, not any one in particular.

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting to make the change effective.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

In Firefox Safe mode these changes are effective:

• hardware acceleration is disabled (Options/Preferences -> General -> Performance)
• all extensions are disabled (about:addons)
• default theme is used (no lightweight theme)
• default toolbar layout is used
• xulstore.json is ignored
• userChrome.css and userContent.css are ignored (chrome folder in profile folder)
• JavaScript JIT compilers are disabled

• plugins are not affected
• preferences are not affected