From MozillaZine Knowledge Base

This article applies to Mozilla Suite.

Bored with the current Mozilla splash screen? You can change it, no programming knowledge required. If you take any Windows bitmap (.bmp) file, rename it "mozilla.bmp" and then drop it into your installation directory, it will become your new splash screen.

Here's a partial list of websites from which you can get premade splashscreens (taken from jcurious' Mozilla page):

See also

• It is possible to disable splash screen altogether.
• You can also make it so that a random splash screen is selected on startup.