Comment 24 for bug 1850064

I still encounter this error with version 68.2.2 on Ubuntu 18.04.3. I also have an upgraded system from 16.04 like @grzesiekc. I removed thunderbird and xul-ext-lightning and then reinstalled.

$ lsb_release -a
LSB Version: core-9.20170808ubuntu1-noarch:security-9.20170808ubuntu1-noarch
Distributor ID: Ubuntu
Description: Ubuntu 18.04.3 LTS
Release: 18.04
Codename: bionic

$ apt-show-versions thunderbird
thunderbird:amd64/bionic-security 1:68.2.2+build1-0ubuntu0.18.04.1 uptodate

$ sudo apparmor_status | grep thunderbird
   thunderbird
   thunderbird//browser_java
   thunderbird//browser_openjdk
   thunderbird//gpg
   thunderbird//sanitized_helper

$ sudo apparmor_parser -R thunderbird
File thunderbird not found, skipping...

$ journalctl -f | grep DEN
Dec 13 20:23:39 MSPC2011 audit[28379]: AVC apparmor="DENIED" operation="capable" profile="thunderbird" pid=28379 comm="thunderbird" capability=21 capname="sys_admin"
Dec 13 20:23:39 MSPC2011 kernel: audit: type=1400 audit(1576265019.934:721): apparmor="DENIED" operation="capable" profile="thunderbird" pid=28379 comm="thunderbird" capability=21 capname="sys_admin"
Dec 13 20:23:41 MSPC2011 audit[28379]: AVC apparmor="DENIED" operation="mknod" profile="thunderbird" name="/dev/shm/org.mozilla.ipc.28379.0" pid=28379 comm="thunderbird" requested_mask="c" denied_mask="c" fsuid=1000 ouid=1000
Dec 13 20:23:41 MSPC2011 kernel: audit: type=1400 audit(1576265021.878:722): apparmor="DENIED" operation="mknod" profile="thunderbird" name="/dev/shm/org.mozilla.ipc.28379.0" pid=28379 comm="thunderbird" requested_mask="c" denied_mask="c" fsuid=1000 ouid=1000

$ thunderbird
1576265020207 addons.xpi WARN Not converting unknown addon type undefined
[28379, Main Thread] WARNING: failed to open shm: Permission denied: file /build/thunderbird-qepI16/thunderbird-68.2.2+build1/ipc/chromium/src/base/shared_memory_posix.cc, line 142
[GFX1-]: Failed to lock new back buffer.
[28379, Main Thread] WARNING: failed to open shm: Permission denied: file /build/thunderbird-qepI16/thunderbird-68.2.2+build1/ipc/chromium/src/base/shared_memory_posix.cc, line 142
[GFX1-]: Failed to lock new back buffer.
[28379, Main Thread] WARNING: failed to open shm: Permission denied: file /build/thunderbird-qepI16/thunderbird-68.2.2+build1/ipc/chromium/src/base/shared_memory_posix.cc, line 142

I already tried settings the permissions on /dev/shm as suggested here:
https://www.linuxquestions.org/questions/slackware-14/thunderbird-60-black-windows-running-as-user-4175637823/

Is this the same bug or a different one and can I add additional diagnosis to clarify this?