> The new UI will remain customizable. It is, and will be, possible to change many aspects of the look and feel, such as icons, colors, font sizes, and spacing between controls through theme plugins. We’ll add options as needed to accommodate user preferences, like the ability to customize the toolbar.
I frequently wish more programs would implement a UI as flexible as the one in Firefox, where you can just drag buttons around, add extra large dedicated buttons for zoom, remove buttons you don't need etc.
Even more flexibility would be cool as well in more specialized software, but I think FF does enough while not overwhelming the user with possibilities.
The community will do the rest.