Due to the layout of my desk, it's not actually within reach -- not without getting up and leaning over my desk anyway. And sure I could turn the volume down, but I'd need to turn it back up anyway because the volume level between videos can, as mentioned above, change from quiet traffic to a howling wind as a car does 100km/h down the highway with windows rolled down.
I like how Windows has a volume mixer. I can turn Firefox down to 1 and I am protected from any unexpected sources of sound, which inexplicably are always played at 100% loudness for no legitimate reason. Real genius design at play there.
Too bad the only way to do that on a Mac is to buy a third-party app.