Your web mail has to provide either POP3 or IMAP in order to use a mail client. I don't know if Yahoo allows this. In GMail, you need to login in to your account and go into the settings to allow either POP3 (most common) or IMAP. Once this is done, in thunderbird you can create a new account and there is a setting to import/use your gmail account. You'll need to create an individual email account in thunderbird for each of the locations you want to pull mail for. You will need information such as user name (email address), password, and port number (if it isn't a default number) in order to create some of the thunderbird accounts for those mail boxes.