That seems to be a known issue with firefox:
https://support.mozilla.org/en-US/questions/918780
Also check this (important)
You can enter about:config in the addressbar of firefox and increase the default volume:
search and select media.default_volume
change 1.0 to 1.5 or whatever you like
Maybe this is a nice workaround for the moment ( if it works at all):
https://forums.fedoraforum.org/showthread.php?302379-Pulse-Audio-(pavucontrol)-saving-volume-levels-for-individual-clients
If all else fails:
To permanently boost your master volume beyond 100%, just to compensate for the cockup of mozilla:
sudo apt install pulseeffects
and choose any of the plugins that allow you to do so.
i.e. limiter, maximizer, auto-gain or compressor
whichever you find the most convenient to work with.
The best solution would be: drop Firefox and start using Brave