I have had this issue for a few weeks myself. The firefox volume in raven drops to around 80% of max (i usually have it at max) whenever a new youtube video plays (the youtube tab is refreshed basically). I can't remember when exactly it started happening though.
The volume for youtube itself is always 100% and the system volume doesnt seem to be affected. Its just youtube videos playing in firefox.
I just tried to reproduce it there mind and i couldn't (system is fully up to date using Firefox 72.02). tried both my headphones and monitor speakers (through HDMI) but it seemed to be ok. I will keep an eye on it: see if there is a more specific set of steps.