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How to reset the COUNTER
If you want to reset the COUNTER, you can do this in the Thunderbird preferences.
Open the "Preferences" (Unix) or "Options" (Windows) interface. On the "Advanced" page, select the "General" tab then click the "Config Editor" button.
Then search for "extensions.quicktext.counter" entry.
The value shows the current state of the counter.
Double-click the entry and change the value to 0.