Option to disable autoplay in background tabs exists in Firefox for so long that I forgot it isn't enabled by default.

Go to about:config and set: media.block-autoplay-until-in-foreground = true

In older versions they used another config variable media.block-play-until-visible .

There is also media.suspend-bkgnd-video.enabled to stop decoding video in background tabs after set amount of time (10s default).