It's weird that I haven't been able to find relevant information anywhere. Basically what I want is a modern vertical layout for the inbox of Thunderbird. That is to say, the message sender and subject should not be crammed into one line, but displayed in a taller row, in the same manner as Apple Mail/Airmail. The current manner in which messages are displayed in Thunderbird under vertical layout makes it very uneasy on the eyes, and I'm not really a fan of horizontal layouts.
Thunderbird:
vs.
Apple Mail:
Best Answer
There isn't an extension that does this, but there's an easy workaround which does what you want:
With a message selected press Ctrl-A to select all messages. The right hand pane then displays all the messages selected, with a summary of their content. (From here you can simply click on a message's subject to go to it).