You need to create 'User Accounts' on your computer for each family member. The first 'User Account' should be set up as 'Administrator'. Others can be set up as standard User Accounts. Each 'User Account' can choose to set a password to access their 'User Account'. Thunderbird is installed once on computer and can run in each 'User Account', which will have it's own desktop and set of default folders. You create the mail account for that persons email address in each separate 'User Account'. Profiles, which store emails etc, are stored within the 'User Account'. This means that if you do not know the password to the User Account then you will not get access to see the emails. All computers are designed to have User Accounts for this purpose of several users being able to keep their documents separate and secure. This is the standard method.