Firefox: There are few things more grating than having music playing in one browser tab while you try to watch a YouTube video in the other. Be Quiet can manage your Firefox audio streams for you so you never have to switch back and forth.
Once the app is installed, it works in the background to silence one audio stream when another begins. For example, if you have Pandora playing in one tab, it will automatically pause your song when a YouTube video starts playing in another tab. Once you pause or close the video, the music starts up again. The effect is totally seamless, and frankly, delightful.
Unfortunately, the extension has only been tested to work with YouTube, Pandora and Last.fm, so this might not cover every edge case, and it won't mute obnoxious Flash ads for you. It's tough to complain though, because it works as it should, and it runs in the background so you never have to remember to enable it. If you're a Firefox user, it's definitely worth checking out.