sigh. The reason Firefox isn't disabling the screensaver when it plays video is because I'm using the snap version and it blocks the D-Bus call. https://bugzilla.mozilla.org/show_bug.cgi?id=1785799 is the bug for it. So now I get to have loads of people lecture me about snaps, and presumably I get to remove it and install the upstream version and work out how to make sure I don't lose any of my tabs or settings. This is all very tedious.