I'd like to have the same resize behavior in Thunderbird as in Outlook.
When the Thunderbird window is not maximized and I arrange the panes: list of messages and message preview pane, they look OK and optimized. But after I maximize the window, the size of both panes are fixed and big gap appears in between them.
Outlook has the smart approach to resize the message preview pane, so the window real estate is used optimal.
The best is to see the images of both programs.
1. Thunderbird not maximized
2. Thunderbird maximized - big gap, preview pane narrow and fixed width