From MozillaZine Knowledge Base

This article applies to Mozilla Suite.

First create a number of files with splash screens that you would like to use. Then create a bat-file (on Windows):

@echo off
NumOfPics=0
for /r %%a in (SplashScr\*.bmp) DO SET /a NumOfPics= %NumOfPics + 1
SET /a ChosenPic="%random% %% %NumOfPics% + 1"
COPY SplashScr\%ChosenPic%.bmp Mozilla.bmp
start mozilla.exe

Put the above batch file ("mozilla.bat") into the main Mozilla directory (the one containing "mozilla.exe"). Also in the main application directory, create a folder named "SplashScr", containing n bitmaps (Mozilla splash screens) named "1.bmp" to "n.bmp". Start Mozilla by running the batch file. You'll get a random splash screen every time.

Thanks to mozby that provided us with this neat little trick.

See also

It's possible to disable splash screen altogether.