If you are worried about this I would turn off the Message Pane when cleaning out your Spam folder. View-Layout-Message Pane. The shortcut for toggling the Message Pane on and off is F8.

Awesome, that works! Thanks! Would still be great if this could be a built in feature, but I understand that might be difficult having folder specific behaviors.

mysarlaccpit said

As you know, merely loading images from a Spam message is an undesireable state of affairs.

No I do not know that. Why is it bad? Or have you changed Thunderbird default settings to make it bad? Thunderbird does not load remote images or run scripts contained in emails, so there is basically nothing bad about opening a junk email. If you use a Micrisift or web mail that loads images on opening then the mail sender can get information from the image load request, especially if the URL's used are customised with say your email address to notify of reads, but that is not really something spammers do. Ebay, Amazon, Facebook, Twitter et al are the big users of those privacy invading tricks. They call it marketing. Spammer just flood the world with messages an hope someone reads some of them.

If you use an email viewer that executes scripts, then there is a chance the email will drop an infection your way. Thunderbird does not run them. This impacts google maps and YouTube links in emails which are based around JavaScript to display video players and enforce DRM, but it is much more secure that having what amounts to a program running when you open an email.

I have no way to mark a message as Not Spam (to move it to my Inbox where it belongs) without having the next message in the pile automatically display and load its crap.

Press F8 and turn off the mail viewing pane if seeing your spam makes you uncomfortable. Press it again when you leave the spam folder.

I can't find any way to stop the automatic next display of a message. It would be great if this were a default behavior in a Spam folder!

What is the issue that such a change would be necessary. As I said above opening spam has very little risk associated with it. mostly about clicking links.

But I would be happy to disable this "automatically display next message" feature globally. Thanks for your help folks. Mac OS 10.13.6, Thunderbird 68.6.0

I would think this addon will probably do what you want. As marking a message as not junk deletes it from the junk folder so if you are not selecting another message none will display. https://addons.thunderbird.net/en-US/thunderbird/addon/deselect-on-delete/?src=search