You do not need a roundcube account to use Thunderbird. However, I think it's fair to say that most people who install a mail client such as Thunderbird do so because they want an alternative to the web-based access offered by their email provider. A mail client, such as Thunderbird, is a tool for working with email. Unlike Yahoo, Googlemail, outlook'''.'''com, roundcube and probably your ISP, it doesn't actually provide an email service; there are no accounts or servers being offered to you here. Thunderbird does offer to take you to selected 3rd-party providers, who generally require a fee to be paid to set up an email account. For most of us, there is no real advantage in following this route, as the free providers (yahoo, google, outlook, gmx etc) are sufficient for most users' needs. If you already have an email account, you can add it to Thunderbird; look for "existing mail account" in the account set-up dialogue. If you don't already have an email account, then you need an email account provider, and Thunderbird may not be what you're looking for. Many of us who use a mail client such as Thunderbird do so because we work with multiple email accounts offered by a variety of email providers, and an email client offers a place to work with multiple email accounts ("addresses") all in one place, rather than logging into multiple websites.