I recently had a customer call to ask if I could setup their email client.
I couldn’t see why they would need this, as they seemed competent enough.
Anyway, when I arrived, I was told that a well-meaning employee had tried to setup Mozilla Thunderbird, but did not get anywhere.
When I started Thunderbird, I was asked for a master password as soon as I tried to do something meaningful (eg complete setting up an email account, or changing the settings of an existing account).
It was annoying, as I could make all the changes I needed, but as soon as I tried to save the changes, I’d get the master password dialog box.
Since nobody knew the master password, I decided to find out how to reset the password.
I soon found the trick:
Start Thunderbird -> tools -> error console -> paste the expression
openDialog("chrome://pippki/content/resetpassword.xul")-> evaluate button
Please note that there is a very real possibility that you will lose any other stored passwords… But this wasn’t an issue in this case, as it was a new account.