Screen turns off while Flash video playing full screen (Firefox Mac)

Firefox Mac 48.0.2

OS X 10.6.8

Flash 25.0.0.163

Hello, it seems I get one problem solved as an other appeared at the same time without having done anything. Until now Firefox (last version possible for this OS) kept the Mac from going to sleep after inactivity. Now it allows. But when I display a video in full screen with the Flash Player plugin for Firefox, the screen turns off after xx minutes as it's set in the OS settings. The video doesn't stop, I still hear the audio, but I have to touch the pad to make the screen turn on again, as if I was not in full screen.