Thunderbird on 20.10 is broken

[Bret Busby]

Hello.

I do not know whether the package maintainer(s) for Thunderbird for
Ubuntu, subscribe to this list, but, Thunderbird on UbuntuMATE 20.10
does not load, but instead, displays an error message;

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

Thunderbird running on UbuntuMATE 20.04, had not presented this problem.

-- 
Bret Busby
Armadale
West Australia
..............