Firefox has started making a piercing high-pitched screech when the "save image" and "file update" panels appear. This is a confirmation or alert sound I suppose. It occurs when saving an image from a right-click menu and when downloading a file. Formerly Firefox used the system default sound for this, and that sound is still working in other programs for similar operations.

How can I fix this? Or alternatively how can I disable the sound during such operations in Firefox (or alternatively system-wide)?