Update your browser list
"Yahoo recommends you upgrade your browser to enjoy all the features in the new Yahoo Mail."
Do you realize that there are more than 3 browsers? Vivaldi (Chrome-based), Pale Moon (Mozilla-based)… In any case, add "not interested" checkbox to that warning or just stop showing it every time mailbox is loaded.
[Deleted User] commented
I will not downgrade my browser (Vivaldi) to one of the three you want. As a long-time Yahoo user, I don't want to change to a different email provider, but if this annoyance continues, I'll do just that, and I'm sure many others will do the same.