Low volume on YouTube in Firefox on Debian Stretch

I was watching a couple of videos on YouTube today, and noticed that the volume was very low, even with the volume controls in YouTube and Gnome turned up to full.

I found the solution was to go to Sound settings and click on the Application tab. Firefox output is via CubebUtils, the level of which was set much lower than the main output volume.

Raising the output level of CubebUtils until it matched the main output volume resulted in a much louder volume, with CubebUtils output rising and falling in sync with the main volume control up to 100%.

I don't know whether the low output setting for CubebUtils was the result of one of the less technically minded users of this computer meddling with the settings (one of the main users is only three!), or the result of an update to Firefox or Debian, but if you too are suffering from the same issue, maybe this post will help.