Thank you, Robert, it might well do. However, I have now stumbled over a deeply hidden but simple fix. The "Sound & Video" -> "PulseAudio Volume...
Thank you, Maurice - that's a clue. Another half day of researching and debugging shows that it's not just firefox - feeding a .wav file to aplay...
Debian 9.0.0 installed fine on the M.2 SSD of my shiny new Udoo X86, and the video performance in firefox 52.2.0 leaves my old host for dead - but...
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