Wow, that's annoying.

Can you associate the music with any particular website you've visited before?

The most commonly used media-playing plugins are Flash and QuickTime. Since sites are allowed to use Flash by default without specific permission, I suspect Flash. Problem is, it is difficult to block a site that you can't see.

A problem was discovered last month with the new tab page: due to some feature of a website that users were visiting, a thumbnail image couldn't be generated. When the new tab page was displayed, Firefox retrieved the site in the background to generate the image and music from the site played. This didn't happen every time, because Firefox wouldn't try to create the thumbnail that often, but it was repeatable if you waited.

One workaround was to clear the site from Firefox's history, since it was obvious which site was at fault (lotto.pch.com). For users that intended to continue visiting the site, its tile could be blocked from the new tab page by clicking the "X" that appears on the upper right of the (blank) tile when you mouse over it.

If you think the new tab page is involved with your unwanted sound, display that page, then zoom out (hold down Ctrl and press - a few time) until you can see all of the tiles -- usually 15 spots are laid out, and you see as many as will fit as your standard zoom level. If you see any sites whose tile displays as blank, that would be a potential culprit. You might make a note of the address, then go ahead and click the "X" to block them from the page to see whether that helps.

(By the way, this problem associated with the new tab page playing sounds when it retrieves pages in the background will be fixed in Firefox 41.)

Another potential workaround is to set Flash to Click-to-Play ("Ask to Activate"). This will delay Flash from starting on a page until you approve it. Sounds loaded in the background should never play on unapproved sites.

To set "Ask to Activate", open the Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. Look for "Shockwave Flash" and change "Always Activate" to "Ask to Activate".

With this setting, when you visit a site that wants to use Flash, you should see a notification icon in the address bar and usually (but not always) one of the following: a link in a black rectangle in the page or an infobar sliding down between the toolbar area and the page.

The plugin notification icon in the address bar typically looks like a small, dark gray Lego block. When the page wants to use a blocked plugin, the icon turns red to alert you to the concern.

If you see a good reason to use Flash, and the site looks trustworthy, you can go ahead and click the notification icon in the address bar to allow Flash. You can trust the site for the time being or permanently.

But some pages use Flash only for tracking or playing ads, so if you don't see an immediate need for Flash, feel free to ignore the notification! It will just sit there in case you want to use it later.

If you want finer control: This extension addresses the issue of only wanting some videos on a page to play: https://addons.mozilla.org/firefox/addon/click-to-play-per-element/ (I haven't tried it myself)