Arthur Quinn wrote:

Firefox 53.0 (installed as an update) on Linux Mint xfce (virtualized
on Mac) fails with the message "this content doesn't seem to be working"

Not on the FLAC test page, but on the BBC news page for video articles,
I sometimes get that error even with a kosher firefox, pressing ctrl-F5
and then play again it will usually work, so seems to incorrectly detect
which codec to use on my browser and the refresh fixes it.