OK, there’s the problem. Splash screen is a cordova plugin so it is ONLY available on the device and not in the browser.
Only develop on your phone
Cordova plugins do not work while developing in your browser, because each plugin accesses a specific API (such as camera, microphone, accelerometer) which is not available in your browser…
You can define a global to prevent the error in the browser. But you need to use a device or emulator in order to use the plugin.
I’d recommend you develop in Chrome if you have to as Ionic don’t support firefox fully.