This is a support forum for Firefox. You will have to contact your e-mail provider for help.

Or are you talking about Thunderbird?

It is Thunderbird. I am a silly old man; thanks

moved to the Thunderbird support queue

At first I thought I had missed the direction to the Thunderbird help service, but when I went back and followed the instructions, I found that the Thunderbird help is lumped with Firefox (and other) help; so my problem persists.

If this is an IMAP mail account:

Right click on the imap mail account name in the left Folder Pane and select 'Settings'. select: 'Server Settings' for the imap mail account.

Bottom right under 'Message Storage' select: 'Clean up (Expunge) Inbox on Exit select: Empty Deleted folder on Exit' click on OK

Close Thunderbird. Wait a few moments for background processes to complete. Start Thunderbird.

Try to Delete message.

If you do not want to empty Deleted/Trash folder every time you exit, you can go back into those settings and uncheck that option.