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Password-protected profile
From MozillaZine Knowledge Base
If you wish to password-protect your profile, so that others will not be able to use your profile unless they know the password, you can do so using an extension listed below.
Note that this will not provide a high level of security, especially from advanced users, but it should be sufficient to keep casual users from accessing your profile.
For Thunderbird there are two different extensions you can use.
*
ProfilePassword puts a simple lock on your profile. See the extension
author's homepage for further information on security limitations when using this extension.
*
DiddlyDo will let you password-protect the whole profile and/or selected folders. The extension author has left the user interface deliberately vague, but see this forum
thread for a few usage hints.