Try Open/Open Message in Conversation from the context menu. It will show all sent and received messages in a thread, in a new tab.

Thank you for responding. So when I do that it does open a new tab with a history of the various back-and-forth emails. Still, that seems confusing as some include responses, some don't etc. It all seems very clumsy. For some reason I recall it all being a lot "neater" (but maybe it wasn't TB I was using), where I just click on the email and all the emails - sent and received - are there in one tab where you can scroll down and read the thread. I guess that is what I'm trying to get. Is that possible with TB?

The view you recall "I just click on the email and all the emails - sent and received - are there in one tab" seems to be what I suggested. Ctrl+Shift+O is the shortcut. You might prefer the view provided by the Conversations add-on.

I see. It's just that I assume when I keep responding to an email, it's all part of the same thread. But for some reason the conversation seems to be broken up with different threads. So there are a bunch of "conversations" instead of just one.

By the Conversation Add-on you are referring to the Open Message in Conversation selection? Because when I do that I basically get the same result as the Ctrl+Shift+O. Or are you talking about an actual Add-on for TB?

Thanks again for your assistance...and patience.

Threading can be broken if the mail app of your recipient doesn't add In-Reply-To or References headers.

See https://addons.thunderbird.net/en-US/thunderbird/addon/gmail-conversation-view/