Thank you for sharing your feedback @JF1
I did a quick search in Bugzilla and found a few common bug reports relating to Firefox's volume mixer. I recommend signing up and adding your issue to an existing bug where this issue has already been raised.
Another possibility is that your problem is related to a compatibility issue with your operating system's audio driver. In that case, you may need to update your audio driver or make changes to the audio control panel in your operating system.
