If you are using the defaults, the message is already marked as read before you attempt a delete. To change that default, Settings > General > Reading & Display > adjust the items at "Automatically mark messages as read".

But more directly to your question, it is not possible.

It is requested at https://bugzilla.mozilla.org/show_bug.cgi?id=545504

Hi, Thank you for getting back to me I see this was discussed 15 years ago! I searched this on line and found numerous posts about this very issue and I think it is from people, Like me, who are thinking of moving from Outlook and would like to keep the process available in Outlook where you would like to to keep the amount of files held in inbox low by filtering / rules and moving to trash(junk), and maintaining unread status in trash(junk) of files that do not merit moving/copying to dedicated folders. It is convenient if a later a "second look" of messages that are filtered or manually cleared quickly maintain their correct status as "unread". I cannot think why "read" is applied automatically when the message has not been "read" even if the "Reading & Display" / "Automatically mark messages as read" configuration is ignored! Coming to Thunderbird from an Outlook users point of view it appears strange and pointless to work this way.

HOWEVER - I note that if you just drag your unread message to trash it works fine! All to much for my simple brain to fathom I guess And they all shout welcome to Thunderbird *!"?head

Anyway your help is much appreciated and I think I have a lot more to learn in my determination to move to Thunderbird Regards Greg