Earlier quoted context omitted.
Presumably the column with the list of emails in the currently selected inbox. (2nd column from left.) Some email clients have multiple rows of information in that column - the first row might have name & subject, the next row might show the first two sentences of the email. Or it might have the subject on a separate line, so the column can be kept narrow but still show the full subject line. Think of the iOS Mail or…
I hope they don't go in that direction, I like my emails compact on 1 line.
It's not doable with CSS only anyway, so it won't happen if they just design a new theme on the current XUL toolkit (correct me if anything changed about that lately).