With one of the recent Firefox releases (current version is 64), autoplay videos began to play again, although they start muted now . None of the previously-working methods work (e.g. about:config media.autoplay.enabled), the documented preference is not there in 64 (promised for 63: either never happened, or was removed). Extensions that purport to disable autoplay do not work.
The solution that does work is set media.autoplay.default to 1.
Finding the working option required a bit of effort. I'm sure this post will become obsolete in a few months, and add to the Internet noise that makes it harder to find a working solution when Mozilla changes something again. But hey. Everyting is shit, so whatever.
 Savour the bitterness of realization that an employee of Mozilla thought that autoplay was okay to permit as long as it was muted.
UPDATE 2019-04-10: They updated Firefox to v.66 "Quantum", right in the middle of F29. The above is not enough now. One must also set media.autoplay.enabled.user-gestures-nee