...but can I patch the look and feel of the user interface, or are we not permitted to patch superficial things that affect "branding" and user experience?
You can create patches that change the interface and/or propose UI changes. They will go through an additional ui-review though. Sometimes the result will be that writing an add-on would be a better fit for a feature or UI change.
Also, if your proposal is around reverting something to a pre-29 state, it has probably already been discussed a lot.
Everything is discussed via meetings and mailing lists. I guess you can not make significant changes to the UI, but you can contribute by writing an extension.
I liked Firefox 28, not Firefox 29 and up.