Rated 2 out of 5 stars
Does not work on TB3.1.
It is already about a year since I e-mailed the developer about this issue and he just claimed that it works fine for him.... (?!) So don't expect this extension working anytime soon.
@joeltx: The "Compact" button doesn't seem to anything here. Would make sense that Thunderbird had this functionality built-in already though.... and it should be right there in the toolbar _by default_.
2 stars instead of 1 just because I remember this working so well in TB2.