Comment 39 for bug 2047262

(In reply to Kestrel from comment #31)
> (In reply to Eugene San from comment #29)
> > Recently, with versions ~120 screensaver/lock inhibition stopped working completely.
>
> It works for me on vanilla Ubuntu 23.10.
>
> The `video-playing` inhibitor is only set when the video is visible (ie window is not minimized or fully covered by another window). Wayland is able to detect covered windows unlike X11 and was recently enabled in version 121. You can disable Wayland to see the difference:
>
> MOZ_ENABLE_WAYLAND=0 MOZ_LOG="LinuxWakeLock:5" firefox

I have not tried Vanilla 23.10 and since other browsers do not have this issue I see no reason to blame/exclude any specific distro. Numerous reports support.
Now when I think about it, I tested both X and Wayland variants of Ubuntu so we can't blame X or Wayland either.
Maybe your routine/setup/hardware somehow avoids the issue and we should figure out how your setup is different from let's say generic install of "Vanilla" system in Virtualbox.

Also, why would you assume anyone taking their time to report the issue here, would test with the video in background or partially covered? It's a possibility, but common we are mostly gown-ups here.