Why does sound fail and/or make noise when I run FireFox on Ubuntu 20.04? However, Brave & Chrome work fine

I am running the current version of FireFox 92.0 (64-bit).

If I start FireFox then the sound on my line out sound does not work:

And, when it does work, there is a lot of fuzz sound : whitenoise as the music plays.

However, when I run Brave this never happens. Have been running Brave for months & I hadn't had any problems. I switched to FireFox again a couple of days ago and the sound problem started happening again.

What I've Tried

I was having the problem where no sound was coming out (with Firefox running) so I quit FireFox and started Brave and it solved everything --- sound started again and it was clear with no whitenoise.

It's probably some driver conflict or something, but seems odd that it occurs when I run FireFox and nothing else causes this problem.