Suddenly, no changes having been made, sound in videos no longer works. Still works fine in IE and Chrome, however.
Although there is no problem in any browser that I've checked except for Firefox, I've done all the obvious checks anyway, but everything checks out fine. And I have not installed anything or updated anything on my system in over a week. I'm a retired systems engineer and I'm stumped.
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See if this article works What to do if Firefox won't play any sounds