This bug is driving me crazy. Watching videos in fullscreen does not inhibit the screensaver/sleep settings on Linux. Whenever I try to stream a movie in fullscreen, my screensaver kicks in at the appointed time, as if I'm just idle.
This bug has been around forever. This seems to be the active entry on Bugzilla.
I LOVE Firefox, and it's a great browser for using on my Linux HTPC. For my purposes, it's much better than Brave. But you know which browser doesn't have this problem on Linux? I'm sorry to say... it's Brave.
I've tried different versions of Firefox: Flatpak, ESR, Trunk. They all have the same problem. When I open them from nothing, they often work great. I can watch a movie fine. But then I close out the movie, leave Firefox open, and walk away. My computer goes to sleep, as it should. The next time I use my computer-- with Firefox still open from earlier-- and try to watch a movie in fullscreen, I get the screensaver/sleep issue.
I really don't want to leave Firefox but this bug (and its longevity) is making it hard.
Until it's fixed, we have Caffeine.