If you want to remove email from Mozilla Thunderbird, you can easily do it but bear in mind that your email in Thunderbird is usually synchronised with the server and your other devices, so if you delete a message on one machine it will be deleted everywhere.
1. Open Mozilla Thunderbird.
2. Open Inbox and right click on the email you want to delete and then click on the "Delete Message" option.
(or in many cases you can just press the delete/backspace key on your keyboard)