Identifying SPAM is usually done to prevent its appearance in the inbox. Since that is what you want, I suggest changing the workflow. Instead of flagging SPAM in Yahoo, do it in Thunderbird. Another approach would be to create a filter that moves SPAM into your inbox: that seems more likely what you want.

I would have to start in Yahoo. It parses messages and sends those it thinks to be spam to the spam folder (obviously). What it seems like I would need to do is have Yahoo basically ignore the fact that anything is spam and send everything to the Inbox... then let Thunderbird download everything. I could sift through what is and isn't spam there. Once everything is in Thunderbird is when I want to separate spam from proper email- not at the Yahoo site, from which I draw my mail.

Thing is, I can't find a way to automatically shunt (again, basically ignore the fact there is a spam folder) spam mail in Yahoo to the Inbox by filter. I know this isn't technically a Yahoo forum, but I thought someone might know a trick for that platform. Once everything is on Thunderbird, I can manage where it goes easily...

Try a filter. The spam folder should be accessible and that could drag it to the inbox.

If the account is IMAP, not POP, you simply subscribe the Spam/Bulk folder in TB to have those messages downloaded to TB along with other folders such as Sent, Inbox etc.