Firefox 4 released few days back comes with new UI and many new features. If you are still not satisfied with the look and feel of Firefox 4, you should be trying out Stratiform add-on for Firefox 4. This add-on can virtually customize almost all areas of UI in Firefox 4. With this add-on, you can customize toolbar buttons, icons, text field. It also allows you to change tabs color and also the tab display style along with dimensions.
The options are neatly grouped and there are 4 main options- Toolbars, tab bar, add-on bar and App menu button. Each of the options have sub menu. You can almost customize every part of the UI of Firefox 4 with this add-on.
You can also customize the color and text of the orange button on the top left corner of Firefox 4.
Stratiform is a nice add-on for Firefox which lets you to customize the look and feel of Firefox 4.