Google has a customized version of the Flash Player built-in to Chrome, which presumably means it can develop features for Flash that other browser makers can't. Until recently, the standard version of Flash did not offer a way for browsers to figure out if a sound was playing from Flash-based media. All Firefox could do was know whether Flash was in use at all.
That's doesn't help much since it would be silly and confusing to put a noise indicator at the top of a tab if, say, Flash was only being used to display a silent animated ad. It also isn't great for the user to only have a tab noise indicator when HTML5 videos are playing--unless, of course, you knowingly wanted that feature by installing the Noise Control add-on.
So Firefox developers were stuck with a Flash problem until earlier this year when Adobe added the ability to detect Flash-based audio. That said, it's entirely possible Mozilla could release this feature with the HTML5 functionality first and then add Flash compatibility later.
Regardless, let's hope we see this feature hit Firefox very soon. I can think of a few sites this would be useful for, and I'm sure you can too.
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