Earmix Tabs by Faheen Ahmed

Set the volume of each tab on its own.

What it does
- A volume slider and a mute button for every tab, in the toolbar popup
- Turn one noisy tab down without touching the others
- The site keeps its own volume controls; Earmix Tabs works on top of them
- Settings stay while the browser is open

With Earmix for Mac (optional)
Earmix is a per-app volume mixer for macOS. With it installed, every Firefox tab that plays sound also appears in Earmix under Firefox, next to your other apps, so you can set tabs and apps from one place. The extension talks to Earmix through Firefox's native messaging, on your Mac only. Earmix is available at https://earmix.app (14-day free trial).

Boosting a tab above 100% and recording a tab need tab capture, which Firefox doesn't offer, so those two features are in the Chrome version only.

Privacy
Earmix Tabs collects nothing. Tab titles and addresses are only shown in the popup and, if you use it, passed to Earmix on the same Mac. Nothing is sent over the internet.