How to disable the warning sound when closing firefox

I appreciate to be warned when I try to close Firefox with multiple tabs open, but I really don't want to hear the warning sound. How can I disable the warning sound for Firefox?

You either have to turn off all sounds for Window notifications or specifically stop Firefox from warning you.

To stop Firefox showing the warning, open the options and navigate to the Tab "Tabs" and uncheck the setting about warning when closing multiple tabs. If you would rather turn off all warning sounds open your indicator-sound in your indicator area in the top right, open the sound preferences and in the tab "sound effects" you can mute the warning sound level.

Using the sound/volume indicator, click "Sound Preferences"

Under the 'sound effect' tab you can mute the alerts, or even turn off all window sounds.