I agree too, it is too much space wasted, I’ve been using compact density from the moment the UI started to get bigger.
I understand keeping track of too many features and customization makes very complicated and expensive to provide support and updates, so it is “natural” for software developers to drop support for the less used ones. But compact density is very useful and I think it is one of the reasons that many who choose Firefox on its early days are still choosing it.
Don’t scare your followers away, Mozilla!