Mozilla in recent years has been flagrantly ignoring the usability costs of their changes and has shown a willingness to make gratuitous changes. Many of these changes could make sense if Mozilla expected their user base to be on an exponential growth trend, such that the existing users who are alienated or annoyed by changes will soon represent a small portion of the overall username. But that's obviously not the situation Mozilla is in.

Considering that the visual processing model of the human brain is based on change detection (hence why animations will get your attention and are distraction), all unnecessary changes do is add mental noise and force reaccommodation, wasting countless hours of trained behaviour. Once a UI becomes familiar, you don't need to pay attention to it and can focus on the content instead. But when it changes, it attracts attention for as long as it takes to changes one's long-term-memory of it.

I haven't done any studies on it, but it wouldn't surprise me if those who hate these sorts of changes the most are also the ones with the best long-term memory and observational skills.