[clum]

When they broke almost all existing Addons (some are gone to this day, like Classic Theme Restorer) with the Quantum Update for instance. Or when they decided to ditch Thunderbird for no reason at all. And then sort of undecided.

[zozbot234]

AIUI, Classic Theme Restorer has become redundant since Quantum brought back the "classic" look anyway. Of course there are many other examples of broken extensions.