Just some terminology first, to be clear: Thunderbird is an email client. It's a program that lets you download emails to your computer, compose them, save them etc., and thus manage your email account. An account is an arrangement you have with an email provider, like Hotmail, Yahoo, Gmail and many other companies, who provide the facility to transmit and store your messages on their servers. If you have Thunderbird already and it is set up with one account, yes you can add another if you wish: many users here have multiple accounts, often with different providers. Instructions on how to create a second (third, fourth ...) account are given here: https://support.mozilla.org/en-US/questions/995656

A similar problem: I am sharing a PC with my wife as one and the same user. She already has TB with her own email accounts, and I need a separate "application" for my two email accounts. Can I have two different TB applications with different DEFAULT profiles? If so, how?