[brymaster]

  Media playback on new tabs is blocked until the tab is visible

^^ That feature was in the Beta release notes but didn't seem to make it into 53 release...disappointing because it appears to work great in Beta/Developer Edition. Can anyone from Mozilla explain why it was scrapped?

[callahad]

The main bug is at https://bugzil.la/1308154. Looks like it's scheduled for Firefox 54 and was accidentally included in the 53 relnotes.