I dont know if your problem stays within Tbird.

If not right-click on tray-basket and chose settings mark confirm on remove

If it's in Tbird open help/account-settings / server settings and check that delete at once is NOT marked

OK, regarding your two suggestions:

1) I think I failed to mention in the OP that I'm running Windows 7. Presumably this is why I have no clue what you mean with "tray basket" and suggest this problem could be system-wide (it is not).

Note that I am deleting from "Local Folders" and there are very few settings for this special folder in Account Settings, but there doesn't appear to be a reset there, either. How can I reset this prompt?

That having been said, I think you might be on to something with your first suggestion, but I'm not sure what. For instance, does Mozilla Thunderbird use a window toolkit (like GTK), and if so, where would I find the settings for that toolkit since Mozilla Thunderbird only provides a shortcut to the program and no other shortcuts during install? Just knowing which toolkit would help in that I could search for it, however, since there is nothing in the "C:\Program Files (x86)\Mozilla Thunderbird" that looks like an environment/window toolkit, I'm guessing that's not going to be the ticket, either. In that case, perhaps there is a configuration file I need to edit, but which one?

Prior to posting the OP, my searching indicated that this feature would require an extension, but I have finally found confirmation it should not. It looks like the feature was added in 2013: https://bugzilla.mozilla.org/show_bug.cgi?id=308690

Here is the link confirming that (currently at the bottom of the page for the previous link): https://hg.mozilla.org/comm-central/rev/8c6a77840184

I know I was using it and I'm sure I accidentally checked the "Do not show this again". The second link above is the diff/patch information for the change. From there, it looks like the preference setting is in "mail/app/profile/all-thunderbird.js" and could be edited there, but I can't figure out what that translates to in Windows, so I'm wondering if it's a source-code file and not part of the application I can edit.

I finally gave up on looking for that and searched for *.js in %HOMEDIR% and ultimately found that this setting resides in the file prefs.js which is located in the profile directory at %APPDATA%\Thunderbird\Profiles.

I can confirm that editing prefs.js while Thunderbird is stopped to change this line: user_pref("mail.warn_on_shift_delete", false); to this: user_pref("mail.warn_on_shift_delete", true); and saving the file and starting Thunderbird resolves my issue. That having been said, I have to believe there is a setting for this in the front end that I just don't know how to find, so I am going to leave this question open in case someone can point me to it. That way, if someone reviews this and finds out there is NOT a front-end setting to change this, a bug can potentially be submitted accordingly.

tray-basket, paper-basket, paper-bucket, TRASH-can. Thats in Windows 7 (My win7 is in swedish, so I have to guess sometimes) And yes its not under help but under tools(?) my mistake.

Presumably I posted my solution while you were typing this response. I did not mark it as my solution because I believe there is a better front-end solution. Knowing you're running Windows 7 in Swedish, I'm guessing the English translation is Recycle Bin (I was previously guessing taskbar). Regardless, the issue was definitely only in Thunderbird and not system-wide in Windows. Moreover, based on the location and structure of the configuration file I had to edit, it was not related to any specific account, but part of the profile configuration. I believe this would imply Tools -> Options is the right place to look, but I still can't find a setting there and don't want to open a bug if there is a setting there or somewhere else. Another obvious and possibly more accurate place just I looked is Account Settings -> (select an account) -> Managed Identities -> Edit, but I didn't find a setting there, either.

There are ways to find your profile-data with help/troubleshooting-information and click on profil-button.

you can edit prefs.js with tools/alternativ / advanced / common (first flap)/ config-utility-button.

To view %appdata% you need to have see hidden files and directories turned on.

Thank you for posting, as this information could be useful to someone else experiencing the issue, however, the issue is only sitting open in case someone else can confirm that there is/isn't a way to change this setting in the GUI.

edit prefs.js with tools/alternativ / advanced / common (first flap)/ config-utility-button.

change mail.warn_on_shift_delete;false to true

Just a q , why dont you use trash/recycle bin? you could set it to empty when Tbird close. That way your still safe without having to confirm.

RE: Solution through GUI, sorry I didn't understand your post at all the first time. The second time it was obvious, and in the English version, I would word instructions as follows: Tools -> Options -> Advanced -> General Tab -> Config Editor From there, one would find and change mail.warn_on_shift_delete back to true as you indicated.

Regarding my use of trash, I use SHIFT+DEL out of habit. While using the Trash and having it empty when closing Thunderbird is probably a good idea (and one I might actually implement), I would still prefer a prompt when I hit SHIFT+DEL (intentionally, out of habit, or otherwise). Thank you for helping me find how to fix the setting via the GUI. I'm sure this could be useful for other behaviors in the future.