Comment 32 for bug 1901829

It sounds like something in the upgrade from thunderbird 68.X (in focal) to version 78.Y (in groovy) is possibly making thunderbird automatically download language pack updates from the extensions site, that shadow the ones that are installed system-wide. This would happen the first time thunderbird is run after the update, which explains why it runs okay once and fails afterwards.

This is just an hypothesis that needs to be confirmed.