Firefox volume lowers volume automatically

Firefox keeps automatically lowering volume to 39%. I open sound settings to reset it to 100% and after I close the sound settings it lowers itself again.

Which Desktop Environment are you using?

I’m using the Cinnamon desktop.

Can’t seem to repro, at least under kde plasma.

Can you check that flat-volumes is set to no or commented out in /etc/pulse/daemon.conf

Yes, it is set to “no”.

Can you check if the system volume is constant? Is that changing too?

The system volume is constant. It stays at 100%. I’ll try a different web browser to see if Firefox is the issue.

I tried opening videos with Brave and several videos later, no volume change. It seems to be an issue linked to Firefox somehow.

Looks like a bug in Firefox.