XML Parsing Error: undefined entity Location: chrome://messenger/content/messenger.xhtml Line Number 905, Column 3: <key id="openLightningKey" --^
This was working properly prior to the upgrade to Fedora 33 but
after installing (via dnf update) the latest version of
Thunderbird (78.3.1).
You can still open it using "thunderbird --safe-mode".
After much searching and guesswork I found a report describing it here:
https://www.mail-archive.com/ubuntu-bugs@lists.ubuntu.com/msg5848655.html
The workaround/fix for me was apparently to remove all the
language packs installed in .thunderbird/(default_profile) as
described here:
https://www.mail-archive.com/ubuntu-bugs@lists.ubuntu.com/msg5848871.html
I added back in the English GB package and it still worked. I note that I had numerous language packs that were flagged as incompatible but removing only those did not help.
Unfortunately I haven't the time/resources to further debug but hopefully this will help anyone else that encounters this issue.
~~R