The threshold for what kind of UI changes are worth complaining about should not be set at drastic fundamental reorganization as you seem to want, nor at the "any visible difference" you impute to Mozilla's detractors. Rather, the threshold should generally be at any change that requires existing users to re-train or otherwise detracts from the productivity of experienced users.

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.