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As pointed out by Fenman, an App Password is required for each third party app or program to access Fastmail, including Thunderbird. In other words, your main account password (used for logging into your account with a browser) can not be used by Thunderbird. The issue must have been as you say some password caching issue with Thunderbird. If you log into your Fastmail account with a browser and look at the Settings > Privacy & Security > Integrations tab, you will see the App Passwords used for Thunderbird, Outlook, mobile phone and tablet email apps, and other devices.
Bill
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