Check that cookies are allowed in TB Options/Privacy, then go offline from File/Offline/Work Offline. Add the account from File/New/Existing Mail Account, enter your credentials, click Manual config. Use imap.mail.yahoo.com on port 993 for the incoming IMAP server or pop.mail.yahoo.com on 995 for POP, smtp.mail.yahoo.com on port 465 for the outgoing server; SSL/TLS security, User Name = email address. If OAuth2 authentication isn't available from the drop-down, click Advanced Config., so OAuth2 can be selected for the authentication in Server Settings. Repeat for the smtp in Outgoing Server (SMTP) at the bottom of the left pane of Account Settings. After the account is set up, go back online, and when you connect to the account, a browser window will appear where you enter your password and the OAuth process will store a token in Saved Passwords.