If only they actually halted major development that would be great.

The problem is that there are Mozilla developers who appear to randomly poke at the codebase and "change" stuff. One recent example is how they've changed the calendar event edit dialog. Now instead of being able to edit the event directly, you have to click on an intermediate button (so first you have a read-only dialog, then the actual edit dialog).

When I've searched why they would do that, I found that it was because one developer thought that "it's probably how it should be". They've changed something that's been that way for more than 10 years with no UX study, no discussion with users - just one guy who thought that it'd be nice to change this.

What you described sounds like a symptom of not having someone in charge leading execution of a consistent vision.

And that’s what exactly happened 10 years ago. Mozilla handing Thunderbird over to the Community to manage.

So I don’t disagree with your frustration, but am simply saying that the issue is caused by lack of centralized ownership … which Mozilla communicated a decade ago they would no longer own.