Can not create new mail message when reading mail message

[PJW]

[Thunderbird 78.11.0 (64-bits)]

When reading a mail message I can not start a new message by using Ctrl-N, and menu option File > New > Message is greyed out.

Used to work under previous Tb-versions.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: thunderbird 1:78.11.0+build1-0ubuntu0.20.04.2
ProcVersionSignature: Ubuntu 5.8.0-59.66~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-59-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
AddonCompatCheckDisabled: False
ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC2: pjw 1020 F.... pulseaudio
 /dev/snd/controlC0: pjw 1020 F.... pulseaudio
 /dev/snd/pcmC0D0p: pjw 1020 F...m pulseaudio
 /dev/snd/controlC1: pjw 1020 F.... pulseaudio
BuildID: 20210528153351
CasperMD5CheckResult: skip
Channel: Unavailable
CurrentDesktop: KDE
Date: Tue Jul 20 21:47:57 2021
ExecutablePath: /usr/lib/thunderbird/thunderbird
ForcedLayersAccel: False
IncompatibleExtensions:
 Enigmail - {847b3a00-7ab1-11d4-8f02-006008948af5}
 TT DeepDark - {9ed238c0-af95-11e0-9f1c-0800200c9a66}
InstallationDate: Installed on 2021-03-26 (116 days ago)
InstallationMedia: Kubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
IpRoute:
 default via 192.168.178.1 dev enp4s0 proto dhcp metric 100
 169.254.0.0/16 dev enp4s0 scope link metric 1000
 192.168.178.0/24 dev enp4s0 proto kernel scope link src 192.168.178.13 metric 100
Plugins: Shockwave Flash - /usr/lib/adobe-flashplugin/libflashplayer.so
PrefErrors: Unexpected character ',' before close parenthesis @ /usr/lib/thunderbird/omni.ja:greprefs.js:732
PrefSources: prefs.js
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=nl_NL.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=78.11.0/20210528153351 (In use)
RunningIncompatibleAddons: True
SourcePackage: thunderbird
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 07/11/2013
dmi.bios.release: 4.6
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: P2.60
dmi.board.name: FM2A75 Pro4-M
dmi.board.vendor: ASRock
dmi.chassis.asset.tag: To Be Filled By O.E.M.
dmi.chassis.type: 3
dmi.chassis.vendor: To Be Filled By O.E.M.
dmi.chassis.version: To Be Filled By O.E.M.
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrP2.60:bd07/11/2013:br4.6:svnToBeFilledByO.E.M.:pnToBeFilledByO.E.M.:pvrToBeFilledByO.E.M.:rvnASRock:rnFM2A75Pro4-M:rvr:cvnToBeFilledByO.E.M.:ct3:cvrToBeFilledByO.E.M.:
dmi.product.family: To Be Filled By O.E.M.
dmi.product.name: To Be Filled By O.E.M.
dmi.product.sku: To Be Filled By O.E.M.
dmi.product.version: To Be Filled By O.E.M.
dmi.sys.vendor: To Be Filled By O.E.M.