I realised today that I'd missed various events that were in a CalDAV calendar that my Thunderbird is supposed to synchronise with.
In fact, the synchronisation was failing, but only by delving into console logs would I ever have known this.
It turns out this bug has been open for 3 years: https://bugzilla.mozilla.org/show_bug.cgi?id=1694709
This is a critical bug. How can a calendaring app just fail to display events, and not warn the user of synchronisation failures?
Yet, while this bug has remained unsolved, countless hours have gone into UI tweaks for "Supernova".
Allocating resource like this isn't OK - critical bugs that severely impact your users must take priority over cosmetic changes.