Airfoil Source Selector is a small application that makes it even easier to connect Airfoil to different applications:
Click the application's icon in Airfoil Source Selector to get Airfoil to hijack the audio. Click on the Airfoil Source Selector icon in the window to connect/disconnect the speakers to Airfoil. Note: the audio source applications (e.g. Firefox) need to be installed in your /Applications folder.
If a sound source application cannot be found when Airfoil Source Selector launches, the related icon will be disabled.