Mute Tab by def00111

Alternative add-ons for Firefox 57+:
https://addons.mozilla.org/firefox/addon/mute-tab-webextension/
https://addons.mozilla.org/firefox/addon/mutelinks/
https://addons.mozilla.org/firefox/addon/auto-mute/
https://addons.mozilla.org/firefox/addon/mute-all-inactive-tabs/
https://addons.mozilla.org/firefox/addon/mute-sites-by-default/
https://addons.mozilla.org/firefox/addon/mutetab/
https://addons.mozilla.org/firefox/addon/auto-mute-hent/

Current tab context menu items:
1) Mute Other Tabs
2) Mute All Tabs