can I have side by side windows w email headers left & content of highlighted email on right?

If you have full headers then choose to see the basic headers FROM, TO, Subject and Date. 'View > Headers' > 'Normal' There is also an addon which can reduce this further. https://addons.mozilla.org/en-us/thunderbird/addon/compactheader/ If you have new emails showing at the top of the email list, then you can raise the 'Message Pane' window allowing more space for the actual message. You can left click and hold down on the top edge of Message Pane to drag upwards. You can change the order by Selecting: 'View'> 'Sort by' > and select Date, Descending and unthreaded/threaded or click on 'Date' column header to sort by Date, if old emails are at the top click on Date column header to reverse this. You could try different layout views eg: 'View' > 'Layout' > 'Vertical'