From MozillaZine Knowledge Base
Many ISPs support both primary and secondary accounts. The primary account has a username/password and a email address, and can be used to manage/create one or more secondary accounts. Each secondary account has its own email address and username/password. Some ISPs impose restrictions on what a secondary account can do/access while others may allow it to do anything a primary account can except create/manage other secondary accounts. However as far as Thunderbird is concerned its just another account. Thunderbird doesn't know about primary and secondary accounts. It only knows about POP, IMAP and local folder accounts.
If your ISP lets you add other user names or email addresses to your account, they're typically letting you create a secondary account even if they don't call it that. However, if adding another email address doesn't involve adding another username/password then its not an account as far as Thunderbird is concerned. Thunderbird doesn't care what email address the message was sent to when it checks for new mail, accounts are specified by the mailboxes location, the username, and the account type (POP/IMAP). You can use Message_Filters to automatically move any messages sent to that address to a separate folder and the multiple identity support to send/reply using that email address if you want to emulate another account.
Many times when a Outlook Express user switches to Thunderbird they ask whether Thunderbird has multiple identity support like Outlook Express does. Frequently this is due to their having secondary accounts and being used to using File -> Switch identities to switch to a different account. Thunderbird has a different concept of multiple identities than Outlook Express. Multiple identities are mainly used to specify the From: address.
If there are multiple users sharing the same Windows account the simplest solution is to create each secondary account as just another account in the same profile. Make certain you uncheck "Use global inbox" when creating each account so that each has their own set of folders. You could also create separate profiles for each user, and use shortcuts to run Thunderbird with the specified profile.