Most folks with issues with certificates and cypher's on Thunderbird.net have an issue with a security software they have installed. Usually their internet security suite.

But why would you be wanting an addon to import Eudora? My understanding is it is not supported in recent versions of Thunderbird. So you need to install an older version which still has Eudora import like V3.

I downloaded Thunderbird 17.0 so I could import. When I clicked on Import,. address book it showed Eudora, but when I clicked on Eudora, and hit Next, received the following message: "No address books were found" When I clicked on everything and hit next, Thunderbird program asked me from where and gave NO options. I tried clicking on just mail and it did show Eudora, but said could not find any mailboxes Still in a quandary

There's an older topic that suggests TB 31 might also work for importing the mail, and the mozillazine article describes how to import contacts if the import routine fails.

https://support.mozilla.org/en-US/questions/1263977

I tried several and none of them worked. I was told that TB 17 would work but doesn't either. I am at my wit's end. My contact list ahs @40 names. I don't want to enter them one at a time.

Does your copy of Eudora actually run and load mail? Eudora import uses registry keys created by Eudora, so just having the data is not enough in and of itself. That is is asking where leads me to think the issue is you do not have a working copy of Eudora.

You could export them from Eudora https://www.lifewire.com/export-your-eudora-address-book-to-a-csv-file-1171220 for import into Thunderbird

Perhaps look at this thread from 2018. https://support.mozilla.org/en-US/questions/1197772 Nothing will have changed as Eudora has not had an update in decades.