Sempre tive curiosidade em saber como trocar o splash screen do mozilla. E olha que estava na “cara” o tempo todo!

^C & ^V do FAQ do mozilla.org:

8.3. How do I change the splash screen at startup?

On Windows, the splash screen is a bitmap image (Windows .bmp format). To use another image, save it as mozilla.bmp into the same folder as mozilla.exe. You can use an image of any size.

Changing the splash screen in Macintosh or Unix/Linux currently requires recompiling Mozilla with the new splash screen included.

Tãããããão simples!