Firefox Splash Screens
You can add a startup screen to Firefox by using the Firefox splash screens provided here plus the Splash screen add-on created by Mel Reyes (aka: mrtech) and Sly Fox. The Splash screen add-on has many different options for customizing the way that your splash screen is presented to you as you can see by the screen shots below.
Downloading the Firefox Splash Screens
You can save the Firefox splash screens on this page one at a time, or you can download all ten of them in a .zip package. Be sure to click on View Full Size to see the actual dimensions of each splash screen. Instructions for using the Firefox splash screens are provided in the .zip.