Join the Mozilla’s Test Days event from 9–15 Jan to test the new Firefox address bar on Firefox Beta 135 and get a chance to win Mozilla swag vouchers! 🎁
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Then reset it.
https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-master-password#w_removing-your-master-password-if-the-master-password-has-been-forgotten