Mozilla Suite themes
From MozillaZine Knowledge Base
Splash screens:
On Windows, you can change the default splashscreen for the Mozilla Suite by creating a .bmp image file, which is then placed in the same directory as the Mozilla executable (Mozilla.exe).
There is an unofficial installer available for the Mac (in the works, not ready yet) [1].
If you want to change the splash screen on Linux you have to build RPM source and change splash.xpm.