Trying to debug that from the forum led me to the test pages https://www.youtube.com/html5 and http://www.quirksmode.org/html5/tests/video.html, which both told me H.264 wasn't working. I checked that ffmpeg -codecs had h.264, and that downloaded files played fine in mplayer using H.264.
Eventually, my searching came across
which says the minimum version for libavcodec.so is 54.35.1. So I found and installed a newer version to go with my two older versions. (I understand from using other A/V programs that ffmpeg's innards change frequently.)
This activated H.264 in the test pages, but I still had no sound, although digging into the second test page I found that
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Eventually, I found an old thread https://forum.palemoon.org/viewtopic.php?f=37&t=9661&p=65944
and did the same syntax transformation to my ALSA config /etc/asound.conf, which had been working fine for everything else. Now it appears that everything else *and* Pale Moon is happy.