when reading an email, Thunderbird displays the reply, reply all, forward, delete, junk etc options at the top right opposite the sender's email address.
In fact it displays the down arrow beside the reply all and the down arrow is actually the same function as the reply all.
Why not change the down arrow to mean 'read next email' i.e the user is finished with the current email and wants to read and process the next email in the date sequence. this will make it easier and quicker for the user to read and process all their unread emails.
Better still, give the user the option to use the down and up arrow icons to mean read the next email downwards or/and upwards in the date sequence
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The code to do this ias already actioned by Thundrbird when a user DELETEs an email they were reading..i.e thunderbird deletes the email being read, and automatically shows the user the next email in the sequence.
So this is an easy and quick request to implement.