At work I have the sound of my laptop not muted because I need sound from time to time. But browsing the internet with Firefox can sometime trigger some undesired sound, very boring in the office. There is the extension Mute Tab to auto-mute a new tab on Firefox so it won’t play sound. The auto-mute must be activated in the plugin options, it’s un-checked by default.
You can find it here, no restart required: Firefox Mute Tab addon
I also use FlashStopper which block by default flash and HTML5 videos, so you can click on it to activate them, it doesn’t autoplay.