One of the popular aspects of Firefox is the way it's open to customisation. There are dozens of extremely useful add-ons which can hugely extend the functionality of the program.
Less useful but often just as popular are the large number of themes available for Firefox. Themes let you 'skin' the program by adding a whole new look to the program's window borders, menu bars, options dialog windows etc.
The theme includes skins for all aspects of the browser, and a custom icon set. The theme is not particularly attractive and certainly renders Firefox less usable.