Firefox 4 daily builds added a new feature that brings better integration with Gnome desktop as it now applies appropriate colour gradients from your current GTK theme.
Have a look yourself:
Firefox 4 now look almost like initial Linux mock-ups:
You can download daily builds (not stable) by running following commands:
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
sudo apt-get update
sudo apt-get install firefox-4.0