Do you have to enable it? I use Firefox Nightly, and have been observing this behavior for a long, long time. It's one of those things that's so minor and yet so important that I wonder how I went without it for so long.
I believe it is enabled by default now, and the value in about:config is `media.block-autoplay-until-in-foreground`
Thanks for the pref name. The feature is currently enabled by default only in the Firefox Nightly channel but you can use the pref to enable it in the Beta or Release channels.