Please change the default window resizing behavior to automatically resize the "Message Pane" whenever it's being shown, rather than the Message List. It's often frustrating when I widen my window to make more room for an email's contents, only to notice that the Message Pane hasn't grown at all.
I generally operate Thunderbird using the "Vertical View" layout, and it's especially odd that the Message Pane just shifts horizontally when toggling the Today Pane (F11), rather than making more room for the pane it appears to be displacing. Unfortunately, the default behavior is equally unintuitive for Wide View and Vertical View layouts--at least for those of us who have the screen real estate and don't like managing emails in separate tabs.
It's likely that most folks who have the "Message Pane" enabled expect the same behavior. And for those who don't, defaulting to the current behavior may be appropriate.