I have turned off all notification sounds, but I still get some startling alerts.
I only get an occasional tone blast - sometimes they are different. I can't figure out what is causing them or how to disable the sound other than turning off the sound.
Alla svar (1)
Firefox itself has no sounds. It must be an add-on or separate program. When does this happen?
Separate Issue; Your System Details shows;
Installed Plug-ins
Adobe PDF Plug-In For Firefox and Netscape "9.5.5"
Adobe PDF Plug-In For Firefox and Netscape 15.16.20039
Having more than one version of a program may cause issues.
You must remove the older programs. Then get the current full installer.
Adobe PDF Plug-In For Firefox and Netscape: Adobe Reader Version 2015.016.20039 Note: Other Software is offered in the download.