There are some things you could change that will be going away - such as having separate (and equal sized) forward and back buttons and separating them from the location field; breaking out the stop and reload buttons; changing button appearance; and the like that depended on XUL and are going away with the new "Proton" UI.

Even now adjustments I used to be able to make in seconds by installing an alternative theme now take an hour of wrestling with the Classic Theme Restorer extension to restore and unbreak with each version upgrade.