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