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To select all emails in a folder:
A list of 'conversations will appear in the lower Message Pane
To completely remove all hidden marked as deleted emails and release space in the folder:
If you would like old emails in a particular folder to be deleted on a regular basis, you can set up a 'retention policy' on a folder. Please note this will delete messages in Thunderbird and originals on remote server.
Let me know if you have any question on it.