ADDENDUM It has been discovered that this behavior is unique to Firefox 32/64-bit on the Windows Pro desktop PC. The Windows 10 HOME laptop, running Firefox 32-bit, does not sleep. EDGE and Chrome don't sleep. WMP doesn't sleep. Running FF in safe mode (no add-ons) changes nothing. Totally uninstalling FF and refreshing it - the same. Older FF versions, when the problem wasn't apparent, now sleep. An annoying, if not critical, situation. ^_^
For years - even prior to the advent of W10 - sleep has never kicked in during Net video streaming (YouTube, for example). Recently, it began to do so, at the specified sleep interval. Even after restoring a system image made when this definitely was not an issue, the sleep while active behavior persists. This hints that an update caused the problem. Virtually all remedies listed in various forums, including driver updates, have been tested. No luck. New ideas welcomed.