You don't get to nominate a place for attachments. There are stored as mime encoded text in the body of the email, just as they traversed the internet.

It is asking me where I want to save attachments. Offline, it lists attachments, but I don't see how to display them. For a sent email with two PDF attachments, examined while I was logged out of Yahoo, Thunderbird has a Save All button to the right of the list of attachments. Clicking it gets the standard dialog listing local locations (much like a Save All dialog in a word processor), so I have to choose a folder or a root. But I don't know where Thunderbird would look for the attachments themselves, so I don't know which location to specify. Is there a standard list of preferred locations that Thunderbird searches in some order?

Nick_Levinson said

It is asking me where I want to save attachments. Offline, it lists attachments, but I don't see how to display them. For a sent email with two PDF attachments, examined while I was logged out of Yahoo, Thunderbird has a Save All button to the right of the list of attachments. Clicking it gets the standard dialog listing local locations (much like a Save All dialog in a word processor), so I have to choose a folder or a root. But I don't know where Thunderbird would look for the attachments themselves, so I don't know which location to specify. Is there a standard list of preferred locations that Thunderbird searches in some order?

have you actually tried double clicking on them to launch the registered helper application? OR ask you for the appropriate one?

Saving the attachment as you describe makes a copy, it does not place the attachment anywhere other than in the body of the email. Detaching the attachment will remove it, but then it is no longer associated with the email at all. It is just another file in your file system.

Okay, I was stupid. I didn't think of double-clicking, thinking they're buttons which wouldn't take double-clicks. But the arrowhead was a contrary clue. And trying would've been worth it. I now tested this on two sent emails, with PDF and GIF attachments, while my Yahoo account was not logged into.

How do you not "log in" to your yahoo account?

Zenos said

How do you not "log in" to your yahoo account?

Zenos. 7 bugs. https://bugzilla.mozilla.org/page.cgi?id=user_activity.html&action=run&from=-14d&who=Nick_Levinson%40yahoo.com

4 support topics. You and I appear to be in all of them. I have just closed most of the bug and not valid. https://support.mozilla.org/en-US/questions/1137884 https://support.mozilla.org/en-US/questions/1143244 https://support.mozilla.org/en-US/questions/1143247

How I not logged into Yahoo: I had already logged out of Yahoo before having Thunderbird do its thing, so Thunderbird would encounter Yahoo as not logged into. It later occurred to me that that could have resulted in a double-login when I didn't need it, and, in an insecure environment where I don't mind using Thunderbird alone, could have exposed my Yahoo account, but it happens that Yahoo doesn't allow Thunderbird logins unless I set Yahoo to accept low-security-app logins, which requires a separate Yahoo login, which I don't do in some environments.

I repeated the above test while I was riding a subway underground with my laptop telling me my Wi-Fi is Not Connected and showing no Wi-Fi icon at the top of the screen. I opened attachments, some not the ones I had tested earlier so they wouldn't be cached, and including PDF and PNG attachments, both by opening directly and by locally saving and then opening. Opening the attachments succeeded, so the attachments are being stored locally, despite the dialog saying it's from a Yahoo domain (I assume that dialog is bugged).

The listing of filings is discussed elsewhere (initially at https://bugzilla.mozilla.org/show_bug.cgi?id=1310459#c3). The issues came up in recent days because that's when I was using Thunderbird.