If you have lost or forgotten your master password or you want to disable the feature without knowing the current password, you can reset the master password. Resetting the master password will remove all stored password information. Upon resetting, you will lose all the stored information in the Password Manager, as this is a built-in security feature to prevent people from simply resetting your Master Password to gain access to your passwords.
Enter chrome://pippki/content/resetpassword.xul into the address bar, press the "Enter" key and click "Reset".
From:
http://kb.mozillazine.org/Master_password#Resetting_the_master_password