How have you tried to delete messages?

I try to mark them in the in box and can not. I pull them up and read but can not delete. I used edge to get in and read and delete. I would like to use firefox

I try to mark them in the in box and can not.

That does not tell me what you do.

There are many ways to delete a message:

• Press the <delete> key.
• Put a delete button in the unified toolbar at the top of the window and press it.
• Right click on a message row in the table or in cards view and press the delete button at the top of the drop-down menu.
• Press the delete button in the message toolbar that appears wherever you view the bodies of your messages (message pane, window, or tab).

Marking messages for deletion in Thunderbird is different from what you have been doing in webmail through your browser. "Deleting" messages actually marks them for deletion and puts them in the trash but no longer shows them in the inbox. So don't look for a "mark" function. Just delete.