Firefox is probably the most central app in that respect, and one of the reasons I moved over to it a few years ago was the constant interface and "features" churn in Chrome.
I'm not going to claim to be the average user, but I am NOT looking for new features in a mature web browser. Firefox has a robust theming and extensions infrastructure for making changes, and changing the container that sits around the Gecko web view shouldn't be done without a good reason (IMHO).