I am actually still going back and forth between chrome and firefox and it's design was something that was holding me back, specially in a stacking window. Now looking at this I looked through themes and found Simple White[1] something that way out dones chrome and et all in sexy factor.
I would like it if something like that was the default and never changed. They seem to keep trying to jump on whatever style is popular when they start a redesign but by the time it appears in a release version it's way out of date. The developer tools are even worse in this regard, they're not even consistent with the rest of the browser. Themes are much better suited to keeping up with trends, the default should be simple.
well styling is always a matter of personal opinion. I don't think Mozilla should spend time on theming it's developer tools as long as it's readable and usable.
For the main theme, as long as it's customizable to satisfy the 5% of grumpy people I'm fine with that. The other 95% just doesn't care.
[1] https://addons.mozilla.org/en-US/firefox/addon/simplewhite/