How to disable drum beat sound when closing FF in Ubuntu 16.04?

Please note, this is the same drum beat sound from login, the difference is this sound occurs whenever I try to close Firefox or Palemoon, and it’s not browser related, because Opera does not play this sound when closing. I’ve posted this question in Mozilla’s forum and someone told me that he has the same problem when receiving system notification, so it seems an Ubuntu configuration and not FF. Anyone had similar problem and succeeded to disable it?
My environment: Ubuntu 16.04, Unity Tweak Tool (don’t know if it has anything to do with the problem), FF 60.8.0esr.