1. Click the ≡ Menu Button and choose Options.
2. Click the Security icon.
3. Uncheck "Use a master password".

• Use a Primary Password to protect stored logins and passwords

If you don't remember your master password, resetting it will discard all saved user names and passwords (as well as personal certificates and private keys, though most people don't have any of these).

1. In the address bar, paste the following address and press Enter.
   • chrome://pippki/content/resetpassword.xul
2. Click the Reset button.

• Reset your Primary Password if you've forgotten it

That does not work on Thunderbird. The box for a masterpassword was already checked arfter installation - it must have come by default because I never entered any password.

How to get rid of it ?

https://support.mozilla.org/en-US/kb/master-password#w_changing-or-removing-the-master-password

Please note that you posted in the Firefox support questions queue. If you have any further questions about Thunderbird please ask a new question in the Thunderbird queue. https://support.mozilla.org/en-US/questions/new/thunderbird