Firefox: How to turn off "beep on error"
Open in Firefox:about:config
Search for:
accessibility.typeaheadfind.enablesound
and set the value to: false
Then restart Firefox.
To prevent any beep, sudo rmmod pcspkr will remove the kernel module that makes beep.
Writing blacklist pcspkr in /etc/modprobe.d/blacklist will prevent the module for being loaded again.