Permission failure then requires that a reject be sent, not that the Promise is left indefinitely unresolved as Firefox is currently doing.
Firefox Autoplay can be used to bybass the Policy: Autoplay is blocked by default (about:preferences). When changing the default to allow for all websites, or specific websites, Firefox will be able to play sounds on non-active Tabs / Windows, if the Audio is played from a Whitelisted website.
It looks like the Autplay functionality only count for Audio, but not WebRTC and SIP/Webrtc Session. It looks like it is still not possible to bypass the strict policy with enabled Autoplay for WebRTC Sessions.