Add an additional row to every message / thread in the message list / thread pane in Thunderbird

Question

I'm trying to modify the thread pane (also known as the message list) in Thunderbird so that every item there has two rows (the goal is to make more space for long subjects).

Here's the default Thunderbird 45.8.0 style:

And here's the layout I aim for (it is the default layout in the macOS Mail app):

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I've leaned that it is possible to modify Thunderbird's UI with the userChrome.css file. Unfortunately, it looks like although it is only capable to changing colors, fonts and so on, it cannot modify the layout the way I want.

Is that true? If there is a way, what should I do?

I'm tagging this question with a Firefox tag as the solution probably requires some smart modifications of userChrome.css, the format of which is used by Firefox as well.

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References:

• XUL tutorial (developer.mozilla.org)
• Similar questions:
  • How to make Thunderbird's message list view show sender and subject on two rows (like the macOS Mail app)?
  • Modern vertical layout for Thunderbird?
• Firefox stylish tutorial (Wayback Machine)
• One of many userChrome.css files (GitHub)
• UserChrome.css (MozillaZine)
• An example of a quite complex userChrome.css file (forums.mozillazine.org)

Answer 1

There is no official support for the requested feature. This feature has been requested for the past 16 years.

See https://bugzilla.mozilla.org/show_bug.cgi?id=213945.