Mater Password bypass
When Thunderbird is opened and prompts for master password, if no password is entered and cancel is clicked three times the master password is bypassed and user has full access to Thunderbird.
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Not full access. You cannot get to the saved passwords in Thunderbird. That is the only function for a master password. It does not protect any messages already downloaded. That is how it works.
Read about a master password and what it does here. http://kb.mozillazine.org/Master_password