Srry I don't see the button to delete the question, because I didn't even differentially diagnose whether it was even Firefox or a webcam program that was running Playing a video in a Firefox tab prevents my desktop from hibernating, even when the video is paused. I'm guessing this is an advanced settings in about:config somewhere, possibly. I multitask a lot so I was partway through a video on utube and paused it, & then my computer was still awake in the morning. The simple solution : don't do that then; or, it's just a little bit of electricy, don't even worry about it. I looked around for solutions in Debian or wherever & they talk about VLC playing, but this was a Firefox tab.