What changes do you want to see? There's so little chrome left in the browser these days that I'm not sure what there is to design besides the appearance of tabs, which is easily customizable.
I'm also curious. What does Firefox do so much worse than Chrome, Safari, or IE? The interfaces of all four are more or less identical.
As a someone who works in the design industry, I appreciate it when ‘graphic design’ takes a backseat where my browser is concerned and they just let the UI merge as discretely with the OS interface as possible...
...and from here, that's exactly what it appears to be doing!
curious, what changes do you want to see?
Here are my comments. There is too much color in the tab and bookmarks bars. It should fade with focus on the content. The icons are too contrasting, the outlive around the folders is too harsh. I don't think they changed the icons design in 10 years or more. I find his lack of aesthetic attention jarring. Same goes for navigation and settings interface.