Flash plugin ignores pulse audio

Ubuntu Studio 16.04 LTS. Firefox 55.0.2, all other software recently updated through Software Updater.

Internal soundcard set to 'off' in pulse audio, using Focusrite Scarlett 2i4 USB. Playing audio from a website with flash player.

Expected: Audio output through pulse audio and Focusrite, just like any other sound from firefox or other sources do.

What happens: Firefox / flash ignores pulse audio and outputs sound directly to internal soundcard. The audio stream does not show in pavucontrol. Volume control is possible only from the flash player.

This error is also reported at https://readlist.com/lists/lists.mozilla.org/support-firefox/34/174788.html

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: firefox 55.0.2+build1-0ubuntu0.16.04.1
ProcVersionSignature: Ubuntu 4.10.0-33.37~16.04.1-lowlatency 4.10.17
Uname: Linux 4.10.0-33-lowlatency x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
BuildID: 20170816210634
Channel: Unavailable
CurrentDesktop: XFCE
Date: Sat Sep 23 20:37:57 2017
EcryptfsInUse: Yes
Extensions: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
InstallationDate: Installed on 2017-03-06 (201 days ago)
InstallationMedia: Ubuntu-Studio 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215)
IpRoute:
 default via 192.168.0.1 dev wlp2s0 proto static metric 600
 169.254.0.0/16 dev wlp2s0 scope link metric 1000
 192.168.0.0/24 dev wlp2s0 proto kernel scope link src 192.168.0.76 metric 600
Locales: extensions.sqlite corrupt or missing
MostRecentCrashID: bp-4ab78069-9aeb-4ec5-a0f2-992b52150208
Plugins: Shockwave Flash - /usr/lib/flashplugin-installer/libflashplayer.so
PrefSources: prefs.js
Profiles: Profile0 (Default) - LastVersion=55.0.2/20170816210634 (In use)
RunningIncompatibleAddons: False
SourcePackage: firefox
SubmittedCrashIDs:
 bp-4ab78069-9aeb-4ec5-a0f2-992b52150208
 bp-b5d7190f-3788-482d-bbe3-ed9cf2140906
 bp-6810ec1c-2408-434e-9aa9-e56e72140830
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 03/03/2012
dmi.bios.vendor: TOSHIBA
dmi.bios.version: 1.10
dmi.board.asset.tag: *
dmi.board.name: QFKAA
dmi.board.vendor: TOSHIBA
dmi.board.version: 1.00
dmi.chassis.asset.tag: *
dmi.chassis.type: 10
dmi.chassis.vendor: TOSHIBA
dmi.chassis.version: *
dmi.modalias: dmi:bvnTOSHIBA:bvr1.10:bd03/03/2012:svnTOSHIBA:pnSATELLITEP850:pvrPSPKAE-00E001N5:rvnTOSHIBA:rnQFKAA:rvr1.00:cvnTOSHIBA:ct10:cvr*:
dmi.product.name: SATELLITE P850
dmi.product.version: PSPKAE-00E001N5
dmi.sys.vendor: TOSHIBA