There should soon be an about:config setting named dom.ipc.processPriorityManager.enabled that will reduce priority of background processes. It's not quite ready yet. You can read about it here:

https://mikeconley.ca/blog/2018/09/20/firefox-performance-update-11/
https://bugzilla.mozilla.org/show_bug.cgi?id=1522879
https://bugzilla.mozilla.org/show_bug.cgi?id=1476981