I love both of my kids, but I don't enjoy it when they are both attempting to converse with me at the same time. Similarly, I love listening to Pandora when I'm at my desk, but I don't enjoy it when it is playing a song when I try to watch a YouTube clip. With the Be Quiet add-on for Firefox, you can prevent two audio streams from bombarding you simultaneously.
After installing Be Quiet and restarting Firefox, the extension will work in the background. If you are listening to Pandora in one tab and start watching a YouTube video in another tab, for example, the extension will pause the song currently playing in Pandora. If you pause the YouTube clip or let it run to its end, your song in Pandora will resume playing from the spot where it was paused. The extension is slick and requires no effort on your part after installing it, but there are a few limitations.
For starters, Be Quiet works only with Last.fm, Pandora, YouTube, and "some sites using HTML5 video." I tried it with two sites I use frequently -- Spotify and Vimeo -- and it didn't recognized either. Also, the extension does not pause ads. That is, it'll only pause Pandora or Last.fm after an ad finishes playing on YouTube and your video begins in earnest. Speaking of YouTube, it should be noted that the extension doesn't work with embedded YouTube videos -- only YouTube.com proper. Lastly, the extension works only when the two audio streams are in separate tabs in the same Firefox window (though it will work in two separate windows as long as one of the windows started as a tab that was dragged out from the other window).
If you regularly listen to Pandora or Last.fm and frequently visit YouTube, Be Quiet saves you the step of having to first pause a song before playing a video and then resuming the song after you're done watching the video. The developer states he is working on adding support for more sites soon, so this is an add-on to keep an eye on.