Firefox becomes unresponsive without freezing after upgrading to 21.04

[Spyros Theodoritsis]

After upgrading to 21.04 Firefox randomly becomes unresponsive without freezing. If you have any video or audio files playing they will continue but you cannot pause them or do anything with Firefox but quit it. Keyboard shortcuts don't work either. It seems that html5 media create this issue.

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: firefox 87.0+build3-0ubuntu4
ProcVersionSignature: Ubuntu 5.11.0-16.17-generic 5.11.12
Uname: Linux 5.11.0-16-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.11-0ubuntu65
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: spyros 4464 F.... pulseaudio
BuildID: 20210318103112
CasperMD5CheckResult: unknown
Channel: Unavailable
Date: Sun Apr 25 13:48:29 2021
ForcedLayersAccel: False
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
InstallationDate: Installed on 2019-01-18 (827 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
IpRoute:
 default via 192.168.1.1 dev wlp2s0 proto dhcp metric 600
 169.254.0.0/16 dev wlp2s0 scope link metric 1000
 192.168.1.0/24 dev wlp2s0 proto kernel scope link src 192.168.1.189 metric 600
NoProfiles: True
ProcEnviron:
 LANGUAGE=en_GB:en
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
PulseList: Error: command ['pacmd', 'list'] failed with exit code 1: No PulseAudio daemon running, or not running as session daemon.
RunningIncompatibleAddons: False
SourcePackage: firefox
UpgradeStatus: Upgraded to hirsute on 2021-04-23 (2 days ago)
dmi.bios.date: 08/10/2015
dmi.bios.release: 5.6
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: X751MA.403
dmi.board.asset.tag: ATN12345678901234567
dmi.board.name: X751MA
dmi.board.vendor: ASUSTeK COMPUTER INC.
dmi.board.version: 1.0
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: ASUSTeK COMPUTER INC.
dmi.chassis.version: 1.0
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrX751MA.403:bd08/10/2015:br5.6:svnASUSTeKCOMPUTERINC.:pnX751MA:pvr1.0:rvnASUSTeKCOMPUTERINC.:rnX751MA:rvr1.0:cvnASUSTeKCOMPUTERINC.:ct10:cvr1.0:
dmi.product.family: X
dmi.product.name: X751MA
dmi.product.sku: ASUS-NotebookSKU
dmi.product.version: 1.0
dmi.sys.vendor: ASUSTeK COMPUTER INC.