How to read and delete emailson server without performing any form of download.
Logon to webmail account and delete the email before you open Thunderbird and download.
via Thunderbird, it's impossible to read an email without downloading and opening it to see what it contains.
It is possible to just select to download headers, then when you select email to read it downloads a copy to a temp folder in order to see what it contains, but it's not downloading a full copy and storing it in a folder.
Imap accounts can be set up on a per folder basis to download headers only or download full copies. Account Settings > Synchronisation & Storage- click on 'Advanced' any folder not selected only downloads headers.