File userChrome.css not working for Thunderbird/Firefox

Newer versions of Thunderbird and Firefox (starting around 2019) disable by default the loading of the 'userChrome.css' file used to configure display preferences. To enable it, you must enable via the Config Editor the setting called toolkit.legacyUserProfileCustomizations.stylesheets.

Reference: https://www.userchrome.org/how-create-userchrome-css.html