Muter

Muter is a Mozilla Firefox plugin that lets you completely mute your browser (and only your browser) with just one click.

Once you've installed it, a small green icon will appear on the bottom bar of the browser. Just click this to mute your browser while maintaining the volume on the rest of your computer. To return to normal, click again.

You can change between two different icons, and, if you want, you can also set up a keyboard shortcut. That way, to mute your browser, all you have to do is use the shortcut keys.

Muter is a very useful plugin since a lot of websites now have advertisements with sound that can be pretty annoying. With this plugin, you can solve this problem quickly and easily.