Auto Mute Plus by Roger Skeie

Auto Mute Plus is a fork of the Firefox extension Auto mute v1.1. and adds the following features:
- A toolbar button to enable and disable auto mute.
- A whitelist (never muted, even with auto mute enabled).
- A blacklist (always muted, even with auto mute disabled).
- A context menu to add domain or URL to whitelist or blacklist.
- Ability to mute all tabs and unmute all tabs from the context menu.

If you don't need the additional features, you can use the original extension https://addons.mozilla.org/en-US/firefox/addon/auto-mute

Description of the original Auto mute extension:
Auto mute of new tabs. Possibility to enable mute tabs in private and normal mode.