No sound in Firefox with HDMI unplugged as 1st peripheral

[Dash]

On a computer having HDMI output, when HDMI is shown as unplugged and HDMI Internal Audio is the first device in the list shown via pavucontrol, Firefox doesn't use next available audio device. Chrome or VLC are dealing OK with the same setup.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: firefox 60.0.2+build1-0ubuntu0.16.04.1
ProcVersionSignature: Ubuntu 4.4.0-128.154-generic 4.4.131
Uname: Linux 4.4.0-128-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: dh 2959 F.... pulseaudio
 /dev/snd/controlC1: dh 2959 F.... pulseaudio
BuildID: 20180607194748
Channel: Unavailable
CurrentDesktop: Unity
Date: Thu Jun 21 14:21:57 2018
DistributionChannelDescriptor:
 # This is a distribution channel descriptor
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-precise-amd64-20130203-1
ExecutablePath: /usr/lib/firefox/firefox
ForcedLayersAccel: False
IncompatibleExtensions:
 Français Language Pack - <email address hidden>
 English (GB) Language Pack - <email address hidden>
 English (South Africa) Language Pack - <email address hidden>
 Default - {972ce4c6-7e08-4474-a285-3208198ce6fd}
InstallationDate: Installed on 2013-10-13 (1711 days ago)
InstallationMedia: Ubuntu 12.04 "Precise" - Build amd64 LIVE Binary 20130203-13:50
MostRecentCrashID: bp-d82a4599-b0db-421e-a6f9-ec0d41180518
Plugins: Shockwave Flash - /usr/lib/browser-plugin-freshplayer-pepperflash/libfreshwrapper-flashplayer.so (browser-plugin-freshplayer-pepperflash)
PrefSources: prefs.js
ProcEnviron:
 LANGUAGE=fr_FR
 PATH=(custom, user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=60.0.2/20180607194748 (In use)
RelatedPackageVersions: browser-plugin-freshplayer-pepperflash 0.3.5-1~webupd8~trusty~3
RunningIncompatibleAddons: True
SourcePackage: firefox
SubmittedCrashIDs:
 bp-d82a4599-b0db-421e-a6f9-ec0d41180518
 bp-e2fac577-1120-4fb7-a01d-c8b062161102
 bp-e938d96b-66a8-483e-8cae-d8f072161031
UpgradeStatus: Upgraded to xenial on 2016-10-05 (623 days ago)
dmi.bios.date: 12/15/2017
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A23
dmi.board.name: 0JNYGR
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 9
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA23:bd12/15/2017:svnDellInc.:pnLatitudeE7440:pvr00:rvnDellInc.:rn0JNYGR:rvrA00:cvnDellInc.:ct9:cvr:
dmi.product.name: Latitude E7440
dmi.product.version: 00
dmi.sys.vendor: Dell Inc.

[Dash (devel-tootai)]

Even worst: just tried to activate again unplugged HDMI which was then second of the list in pavucontrol, Firefox uses HDMI audio and not the internal audio device ! So only switching off HDMI gives sound back in Firefox.