There's definitely some on here that are a waste of resources... https://killedbymozilla.com/

Here's something done badly - Firefox View's Tab Pickup. Because as we know, no one ever opens more than 3 tabs. So that isn't used by many because of that and unless it's implemented into a proper list with actions against it then it will die a death of underuse.

I'd absolutely love to see Mitchell Baker do a Reddit AMA and actually answer inconvenient questions because there would be plenty of them.