@ellenor2000 @thunderbird It might cause some headaches for some people (there definitely will be some very angry loud (minority)), but as far as I know, the most controversal UI changes (density, unified folders etc.) can be reverted in settings, because the Thunderbird team actually listens to user feedback (unlike some big companies).
@ymstnt @ellenor2000 Indeed. in fact, we're not even defaulting to the view you see in these screenshots. So it should still feel very comfortable to people who are used to the existing layouts.