Electrolysis isn't enabled by default
It is enabled by default, you just may have a plugin installed that is blocking it. The plugin(s) could be multiprocess compatible but unmaintained and missing the multiprocess=true flag in its package.json. 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?
https://bugzilla.mozilla.org/show_bug.cgi?id=1551990