This is the biggest headache I have since moving from Outlook.
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Similar idea here about being able to customize keyboard shortcuts on Thunderbird: Customizable keyboard shortcuts for Thunderbird
When I started using Thunderbird, this worked. Esc key closed tab (or message window). At some point, it stopped working. I substituted the extensions (both now broken) "Close Tab on ESC" then "tbkeys-lite". At this point, neither of those extensions is normally available or (once found) works. Can the original function (close tab on ESC) be returned? Please?