I do things differently, but you might find something useful in how I try to make it as efficient as possible. I have 9 accounts but get most of my new mail in one account. If I do get new mail in the other accounts it tends to be very bursty, so I rarely have to look at new mail in more than two accounts at a time.
I enable the message pane (F8 or view -> layout->message pane) and use it to read all mail. With my right hand I use the mouse to select a message, read it in the message pane and then use my left hand to press the Delete key. Pressing the delete key has the side effect of moving me to the next message so I actually don't need to use the mouse for every message. I find this the most efficient way to read and delete many messages. One other advantage over reading messages in a new tab or window is it makes it easier to keep more context since you still see the folder listing. It's also easier if I want to move a message since the folder pane is still visible/accessible - I can just drag and drop.
You could use the delete button on the toolbar in the message pane or the main windows toolbar instead but that is much slower as you need to move the mouse. I also like to use the CompactHeader add-on with it configured to display all of the headers of the message I'm reading in just one line. That helps provide more space to read the message. I can still see the sender, recipients, tags etc. for the message in the folder listing above the message without having to move either hand, so using only one line for the header doesn't cause a problem.
I don't use unified folders, I use view -> folders -> all. I like to have a few virtual folders (created using saved search per
http://kb.mozillazine.org/Saved_Search) that contain all messages that meet certain criteria instead (such as all notifications of replies to any posts I make in these forums, or all messages sent by a few people). The virtual folders don't change where the messages are physically stored, but any actions you take (such as deleting the message) effect both the virtual folder and the folder where it is actually stored.
Almost all of my accounts are IMAP accounts. I use "When I delete a message -> Just mark it as deleted". That just draws a line through the messages headers in the folder listing when you delete it, and doesn't prevent you from continuing to read the message. It doesn't move the message to trash. When I'm all done reading all of the new mail I have the chance to change my mind and undelete a message by deleting it again. Much quicker than having to review the contents of the trash folder. I then right click on the inbox folder in the folder pane and select compact. That physically removes the deleted messages.
