The reality is I already gave up trying to get audio out from Firefox direct to Alsa. All new versions v52.x and up including ESR versions have been modified to support pulse-audio without exceptions, although they haven't yet removed the alsa code from the sources in case they find a volunteer to fix and maintain it..
I was never one to go with bloat when you can configure Alsa to work alone. However, knowing pulse audio is the way to go in future, I'm happy to configure my sound with pulseaudio as it provides greater control of audio streams in terms of path and gain, besides it works great with HTML5 and latest Flash. Support for both are needed for modern websites.