After upgrade to 20.10 Thunderbird stops with parsing error

When starting Thunderbird in Ubuntu 20.10 an error message is displayed

XML Parsing Error: undefined entity
Location: chrome://messenger/content/messenger.xhtml
Line Number 905, Column 3:
  <key id="openLightningKey"
--^

and nothing more happens. However Thunderbird can be started in "safe mode" with the command line command

thunderbird -safe-mode

I have a number of language packs installed, German, English (CA), English (GB), French and Swedish.

How can I make Thunderbird start OK from the apllications menu?

I was experiencing the same problem just earlier after installing an official language pack by Mozilla. After closing and restarting Thunderbird, I was shown a popup with the exact same message you posted, and couldn't get Thunderbird running again.

Based on messages in this (German language) forum thread: https://www.thunderbird-mail.de/forum/thread/85658-thunderbird-startet-nicht-bzw-gibt-fehlermeldung-nur-im-safemode-m%C3%B6glich/?postID=468615, others are having the same problem, also after having recently installed or updated language packs.

I managed to solve the problem on my end by moving the offending file ([email protected], in my case) out of Thunderbird's extensions directory. On my system, all my Thunderbird settings, including the exensions directory, reside at /home/MYUSER/.thunderbird/RANDOMSTRING.default.

I managed to work around this issue on Ubuntu 20.10 by starting with thunderbird --safe-mode and removing all language packs (menu "Add-ons" -> "Languages"). Note that on my system I wasn't able to remove all of them since some of them were installed from Debian packages. Turns out the latter weren't causing any problems, though, and I was able to start Thunderbird without safe-mode.

In case you want to remove the language packs installed from Debian packages as well you can do so by running (adjust according to the languages installed on your machine):

$ apt purge thunderbird-locale-de thunderbird-locale-en

After doing so the list of language packs should be empty and you should be able to start Thunderbird normally.

The accepted answer did not helped me but this did:

1. start thunderbird in the command line/shell/terminal with thunderbird --safe-mode
2. choose "disable all add-ons" and
3. click the button "Make changes and restart". Then it can be started as usual like using the the normal thunderbird start icons or thunderbird in the command line/shell/terminal.

Maybe this is a more elegant solution for users that do not want or can move files in the configuration directories.