I have an onboard sound card, and also a connected bluetooth headset. I have configured the bluetooth device in /etc/asound.conf:# cat /etc/asound.confpcm.bluetooth {    type bluetooth    device 12:34:56:78:9a:bc    profile auto}ctl.bluetooth {    type bluetooth}By default, the onboard card is used for all sound (apparently, the default onboard card does not even need to be listed in asound.conf)When I want an application to use my bluetooth alsa device, I have to specify it, such as:mplayer -ao alsa:device=bluetooth file.mp3That's fine for me. But I need a way to tell my browsers to use bluetooth alsa device as well.I have found a way how to start chromium using the --alsa-output-device commandline option:chromium --alsa-output-device=bluetoothI need a similar way to start firefox, but I could not find any.How can I tell firefox to use my bluetooth alsa device, without having to modify /etc/asound.conf or ~/.asoundrc every time ?UPDATE:I have followed @lgeorget's advice and my /etc/asound.conf now looks like this:pcm.!default {type plugslave.pcm {        @func getenv        vars [ ALSAPCM ]        default hw:0,0    }}pcm.bluetooth {    type bluetooth    device 12:34:56:78:9a:bc    profile auto}ctl.bluetooth {    type bluetooth}When I start firefox using ALSAPCM=bluetooth firefox, I do get sound in my bluetooth headset, but firefox runs at 100% CPU (on my 4 cores) and the youtube video plays at 10x speed (and the sound is correspondingly (garbled). I don't understand what's happening. When I start firefox without ALSAPCM=bluetooth, everything is OK, and sound plays on default alsa device. 
tell Firefox to use another ALSA device
audio;firefox;alsa;browser