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 or unmute all tabs from the context menu.

https://addons.mozilla.org/en-US/firefox/addon/auto-mute-plus/

If you don't need the additional features, you can use the original extension (download link at the bottom of this page).

Below is the original README.md from https://github.com/WaldiPL/autoMute

Auto mute

Auto mute of new tabs. Possibility to enable mute tabs in private and normal mode.

Requirements

Firefox 45+

Permissions

Tabs
Storage

Download

https://addons.mozilla.org/firefox/addon/auto-mute?src=external-github