Works almost as intended. I blocked Youtube for starters and as long as I'm actively interacting with or giving focus to firefox, the counter is ticking down. The moment I'll click on something on my second monitor (in this case, Discord) the counter stops after two seconds. This has been reproduceable. I'll use it further as a reminder for myself, but sadly it's not working as I need it right now :/

Edit: Thanks! Is working as wished :)