I was hoping to see this pushed out by default because I agree that it's the best 'new' feature I've found in the browser in some time.
You can change it in about:config, searching for 'media.autoplay'. I've changed the following:
1) Set 'ask permission' to true
2) Set 'block webaudio' to true
3) Set default to 2, which is 'ask per domain'. As I understand, the default of 0 is 'autoplay', 1 is 'block' and 2 is 'ask' but your favourite search engine of choice can provide clarification on this and the other settings. 2 will cause a pop-up beside the URL bar, like with password save popups, letting you accept or block and with a checkbox to set it permanent.
Apparently, setting both “ask-permission” and “enabled.user-gestures-needed” to true then enables a setting in the privacy area of settings to have a dropdown box with these options. I’m guessing it maps to the 0/1/2 setting I mentioned earlier
I changed all the media.autoplay settings from the default except block-event.enabled, with default set to 1. With that it seems to do what I want. I'm not sure if the strange media.block-autoplay-until-in-foreground behavior described in the comment is still happening but I set that to false also.
1) Set 'ask permission' to true
2) Set 'block webaudio' to true
3) Set default to 2, which is 'ask per domain'. As I understand, the default of 0 is 'autoplay', 1 is 'block' and 2 is 'ask' but your favourite search engine of choice can provide clarification on this and the other settings. 2 will cause a pop-up beside the URL bar, like with password save popups, letting you accept or block and with a checkbox to set it permanent.