change image

Not without recompiling the launcher. You can easily disable it, though, with a simple INI file. Just create a file in the same directory as FirefoxPortable.exe called FirefoxPortable.ini and add this to it:

[FirefoxPortable]
DisableSplashScreen=true

If you ever forget, this is detailed in Other\Source\readme.txt, which is linked to right in the help.html file included with all of our apps.