That's not a Firefox feature. Do you have any addons installed, that may have added that sound?

Is your "new tab page" customized? Just wondering whether it's a feature of the page you're using.

No customizations or add-ons. Firefox 33.0

Is it only when you use the new tab button or command+t, or even if you direct a link or bookmark to open in a new tab?

Could you test in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions, hardware acceleration and some other advanced features of Firefox. More info: Use Troubleshoot Mode in Firefox.

You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

Not all add-ons are disabled: Flash and other plugins still run

After Firefox shuts down, a small dialog should appear. Click "Start in Safe Mode" (not Reset).

Any difference?

That solved it. Thanks! Still don't know why it occurred though.

EDIT: uh, no it didn't solve anything. Still getting that little tune. Driving me insane.

You get the tune while you are in Safe Mode, or after you quit Firefox and start up in normal mode again?

Both in safe mode and normal mode.

That means one of your add-ons is doing this. Check them all.

Hi JLMAGNUSSON, I'm not familiar with Mac, but on Windows, there are some system level sounds that can occasionally affect Firefox, for example, making a clicking sound when a new page loads (navigation sound). Can you find these settings on Mac or in Safari and listen to what is there and see whether any of them match what you are hearing in Firefox?