As it says above, I did my system upgrade this morning, and noted the message advising that firefox should be restarted to get mplayer-plugin working. I did that, but still had no mplayer-plugin - in fact, I had no plugins at all. So I restarted X, and then firefox, and everything was back to normal.
I'm running xorg7, so that might have something to do with it. Anyway, maybe this will be useful to someone else.
TomK: This is because the firefox plugins folder has moved from /opt/mozilla-plugins to /opt/mozilla/lib/plugins and a new /etc/profile.d script has been added.
For this reason, to update the proper environment variables, after updating Firefox to the newest you need to logout and log back in (or restart X, if you source /etc/profile in your X startup scripts.)
Thanks Cerebral - in that case, the message echoed by pacman is not completely accurate. Is it worth a bug report, do you think?