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 Android GMail interfaces, for example.
There's a sane responsive interface solution to that: you get single-line layout on wide screen, and a compact multi-line layout on narrow screen. Answering a nearby comment, it's doable in CSS-only[1].
This entire discussion is not about complex interface design and the lack of technical solutions, but the stagnating product. And about questionable actions to give it a peppy appearance.
I am pretty sure we could come up with something like a switch in the option.
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).
