mozplugger-2.1.4 added
https://wiki.archlinux.org/index.php/browser_plugins
run: mozplugger-update as a normal user then edit /etc/mozpluggerrc (MPlayer -> Mpv):

define(MP_CMD,[mpv -really-quiet $1 </dev/null])
define(MP_EMBED,[embed noisy ignore_errors: MP_CMD(--autofit=$width -wid $window $1)])
define(MP_NOEMBED,[noembed noisy ignore_errors maxaspect swallow(mpv): MP_CMD($1)])
define(MP_AUDIO,[mpv -really-quiet $1 </dev/null])
define(MP_AUDIO_STREAM,[controls stream noisy ignore_errors: mpv -really-quiet $1 "$file" </dev/null])