You can but then there is no privacy. You will be able to see all of his emails and his paswords and he will be able to see all of your emails etc. If both of you agree that this is not an issue then go ahead and add another mail account. To add another mail account to your Profile Folder. * File > New > Existing Mail account or * 3bar menu icon > New messages > Exisitng mail account Usually, if more than one person uses the same computer, you create 'User Accounts' on the computer and set the main person as Admin. Control Panel > User Accounts. User accounts can be setup to use a password. Each person logs on to their User account. Thunderbird is a program on the computer and therefore will be available to all users. Then open Thunderbird in his User Account and create the mail account. This means that your Profile fodlers will be stored within your User Account and his Thunderbird Profile Folder will be stored within his User Account. This means he will not see all of your emails and you will not see all of his stuff.