I've got a bluetooth adapter which works fine as a pulseaudio high fidelity playback (a2dp) device in Ubuntu 17.10. The speaker test works fine in the standard sound settings GUI when I connect to bluetooth using headphones. The problem is that Firefox YouTube videos don't use the bluetooth adapter.. they consistently play over the HDMI speaker.
I installed kmix and it says that Firefox is using a Playback Stream - specifically 'Firefox: AudioStream'.
If I vary the volume of the HDMI audio in the kmix Playback Devices tab, it varies the sound level of the YouTube video playing over the HDMI speaker.
The bluetooth headphone also has a slider at max volume in this tab, but the headphones do not receive any sound from the browser. When the headphone slider volume is changed, a pleasing 'thwop' sound is heard in the headphones.
Is there a way to get Firefox to output to the bluetooth adapter?