I know that you folks are not Mozilla itself, but perhaps you can pass on my complaint as well as answer my specific questions, which I will ask first:

Using Mozilla and Thunderbird in the same sentence is not really all that helpful. Mozilla do not make Thunderbird.

Ever since a recent upgrade, Thunderbird has decided to display entire chains (conversations) of back and forth emails.

That is called threading. See https://support.mozilla.org/en-US/kb/message-threading-thunderbird

But, the order in which they are listed makes no sense to me. In fact, some emails are not even individually listed in my inbox when they are a reply to a prior email.

<bockquote>

HOW do I shut off this craziness: 

See https://support.mozilla.org/en-US/kb/message-threading-thunderbird#w_changing-default-threading-mode

When I click to read an incoming email, that is the only one I want displayed in the preview box.

There is no preview, there is a reading pane, there is a reading tab and a reading window. They all perform the same function.

(If I want to see a list of all emails from a particular client, I'll type their name in the quick search box...like I've been doing for more than 10 years).

This is not a list of emails from that client, it is a related correspondence thread on the same topic. Unless one of you does not know how to click write and start a new thrread when the topic changes.

2nd Question: Again, after some recent upgrade, Thunderbird occasionally sends an incoming email into SPAM, even though the sender might be someone I been accepting emails from for YEARS...and I have NOT changed my Thunderbird instructions in regard to what is or is not spam. </bloockquote> nor has Thunderbird changed how it determines spam in the last decade probably. What I generally see are people moving to IMAP accounts and not being aware their mail provider is now classifying mail as spam as well. Although there is a group of anti virus products out there that mess in scam and SPAM detection. Usually badly. If the person is in your address book and you have that selected as a white list in your junk settings for the account, Thunderbird will not consider the mail SPAM even if it is.

My complaint is that Thunderbird should NOT be making assumptions about what is or is not "good" for me.

the junk filter has always relied on training from the user to make it's early decisions. After 10 years I would guess it is pretty well trained based on your junk not junk decisions over the years.

See https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages

If it adds a new option to the program, it should not "turn it on" automatically.

in principal I agree with you. But I am also in favour of new version wizards that walk you through the new features and offer to turn them on.

Every time this occurs I am, pardon me, screwed up. What would actually be helpful would be an email explaining the option with instructions on how to implement it. Please pardon the long email, but Thunderbird has totally frustrated me recently...

As you have no doubt gathered, I am not aware of any changes to how Thunderbird processes junk in the last decade. The threading is fully documented in the knowledge base and I have linked to that documentation.

The C++ source of the junk filter is here https://searchfox.org/comm-central/source/mailnews/extensions/bayesian-spam-filter/nsBayesianFilter.cpp If you hold your mouse over the darker bands in the left you will see the date of the latest changes and links to the bugs involved with that code. I did not check expensively when I said I was not aware of change. But most of what I looked as was a long time ago other that reformatting of the actual code.

Thanks for your help. Some problems at least partially resolved as a result of your comments, and thanks for taking the time to respond to each of my questions or comments. But, here are a few comments back:

First of all, Mozilla and Thunderbird are related, even if Thunderbird help is from "the community". Mozilla created Firefox & Thunderbird, as well as the Mozilla Foundation, and Thunderbird is operated by a Mozilla subsidiary MZLA...so don't blame me for taking out my gripes on them.

Second, when I type a sender's email address into the quick search toolbar it brings up ALL emails received from them, regardless of what the topic of the emails were. On the other hand, if I have that quick search set up to also search the content/body of the emails, I can type in some text from one email, and it would bring up a list of all emails (even from other senders) containing that text....all of which works perfectly for me.

Third, the pane below the inbox IS referred to by Thunderbird itself as the "preview pane". All I want to see there is the actual content (body) of the specific individual email I've highlighted in the inbox. But, with that "threading" option, not only does it open every email in the thread in a new window, it also shows a bunch of them in the preview pane. I don't want ANY of that. As I said, the quick search bar helps me find every email from a particular sender, and they are nicely listed chronologically.

Forth: Since this threading option is so critical, is should be able to be turned on or off via a "botton" on Thunderbirds "front page"...as easily as clicking "replay". Instead, I was forced to dig deep into settings just to chang a "1" to a"0".

Fifth...Even worse, I fear that once emails are "threaded", it might be partially permanent. In other words, previously, when I clicked a sender's most recent email, almost 10 new windos opened, and the preview pane showed them all. Now, after the suggested change, I STILL get 2 windows opening and two emails showing in the preview pane. Its as if the thread was actually a "thread of threads", and only one level of threading was turned off. (I had a feeling in my gut that this might happen when the tutorial had me search for xxxxxxxxxxxxNewFlags). I don't want ANY threading at all. It's like THANKS, BUT NOW THANKS! Is there any way to accomplish that???

Last, Matt: I promise you that I've made absolutely any changes to my spam filtering settings at all, nor have I moved any emails of valued senders to Spam...so the spam filter(s) should not have "learned" that I no longer want emails from those senders to land in my inbox. This was spontaneous action of Thunderbird, and it has, pardon me, f**ked me royally by my having missed emails that, in a million years, I would not have expected such emails to spam...after years of them going to "inbox".