Why does Page Down/Up using Fn+Down/Up not work on some applications? I have a Macbook. Fn+Down/Up keys work as Page Down/Up in some applications but not in every application.

Some critical applications that this key binding don't work are Firefox and IntelliJ Idea. 

Is there any way to make this key binding work in those applications?
I just tested in my FF ver. 29.0.1 with Fn arrow UP and Down and it works on my MBA 10.9.3

More information on FF shortcuts.

For FF you can also customize the shortcuts.

I do not have the IntelliJ to test.

Some applications use the own keyboard shortcuts and override OS X settings.