I note that now Thunderbird prompts me at start up with a message from Yahoo requiring a digital certificate. Before this issue, I didn't have any currently valid certificates but that didn't seem to be a problem then. However, I just bought a new digital certificate, supplied that new valid certificate in response to the prompt from Yahoo. Then I was prompted for my password to the Yahoo server. Once I provided that, Yahoo once again connected and I am once again receiving email from Yahoo in Thunderbird. I am not sure whether this is a problem with the latest version of Thunderbird, or a change of policy for Yahoo - but that is what I needed to do in order to get my Yahoo email into Thunderbird again. I imagine that others might encounter this same problem.