That is the normal window when the authentication method on the account is OAuth2, as it is with Yahoo, gmail, AOL etc. Click Next and enter your account password to allow TB access to the account. Cookies must be allowed in TB Preferences/Settings.

Thank you SFHowes for taking the time to answer my question. Appreciate it. Best, Douglas

I also posted about this same issue. What was odd to me is that it just started happening. I've been using TB to check Yahoo for years. I didn't change anything in the configuration so was curious as to what caused it to now start prompting?

AZBigDog, yes, same here. Years using TB and this never happened before. What got me was the strange email address on the URL of the Pop Up. I remember though that Yahoo had been sending emails a while ago about closing the accounts or something like that because of lack of....activity? On that same account, which is not used often, whenever I did get mail Yahoo would always send me an email right away asking me to upgrade my experience. Anyway, I did what sfhowes said and the problem hasn't appeared again. Weird though. Those big companies. Always wanting more control. Screw em. That's why I switch 20 years ago to Firefox and Thunderbird.