How to disable sound when switching Firefox tabs with ⌘1-⌘9

I use the keyboard shortcut ⌘1-⌘9 to switch to tabs in Firefox. In OS X every time I do this there's a sound, which can become annoying when the volume is turned up.

I looked in about:config for anything matching sound but wasn't able to figure it out. Does anyone know how to disable this sound?

Solution 1:

That’s certainly not a Firefox sound, at least not one that I’ve heard in the past.

• Do you have any Extension? Try disabling them all for the sake of testing.
• Any other application running that could be using the same shortcut? (Safari uses that to access the quick bookmarks where ⌘1 corresponds to the 1st on the left (next to Top Sites) and so on. However, the lack of bookmark, doesn’t produce a sound on my system. I even have “Play User Interface sound effect” checked in Sound Preferences pane.

My bet is that you have something else running and that is causing the “bonk”.