Works for YouTube tabs;
but does not work for Firefox's Reader View - read aloud.
Also does not work for Speak Selected added to Firefox context menu.
Review added 2026-01-30 for Ubuntu 24.04.3 LTS ; snap info firefox = 147.0.2-1 latest/stable

Doesn't seem to remember volume per-site (even though it is saved in the settings) (also for some reason the settings display for me in German)