Mozilla Suite themes
From MozillaZine Knowledge Base
On Windows, you can change the default splash screen for the Mozilla Suite by creating a "mozilla.bmp" image file, which is then placed in the same directory as the Mozilla executable ("mozilla.exe").
If you want to change the splash screen on Linux, you have to build the RPM source and change the "splash.xpm" on Linux.
To change the splash screen for Mozilla Suite on Mac OS X, use Custom Splash Installer, a third-party software program.
See the Themes article.