Adobe Scout gives me insight into performance problems on a number of games!
Sometimes a game performs poorly on FireFox. I have read elsewhere that FireFox rendering is slower than Chrome, Internet Explorer, and Safari. Any insight as to why? Any tips on what can be done to help when "Waiting for next frame" time exceeds the expected duration of a frame? For example, at 30fps, expecting 34 ms per frame, sometimes in FireFox waiting for next frame on one frame will be 50 ms or higher. In Chrome the waiting for next frame is less than a whole frame's expected duration.
As a minor request, it would help see when the frame rate has become unstable due to activity outside of the Flash Player if the dark gray color of the bar in the profiling view were not so dark. The dark gray distinguishes from costs within the Flash Player and I prefer the dark gray to a light gray, however it would help to clearly see when the frame rate has become unstable even if from activity outside of the Flash Player.
If your problem is "waiting for next frame", then this is nothing to do with your content. Make sure that you're not running other SWFs at the same time, and that you're not running other CPU-intensive applications on the computer. I'm assuming from your description that the waiting for next frame time is consistently driving up the framerate (i.e. you're not getting 30fps because of that) - if it's just the odd frame that goes up to 50ms, and you're meeting your framerate on average, then this is perfectly within the realm of OS/browser timing deviations, and it's nothing to worry about.
Since your problem is specific to Firefox, there may be a bug in the way Flash Player and Firefox interact. If you can reproduce the problem, it would be helpful if you could file a bug with as much information as possible (https://bugbase.adobe.com/), so that somebody can look into it.
I'm afraid I can't really say what the cause of the problem might be - there are a lot of moving parts here, so we'd need to investigate what the specific issue is (I suspect it's related to the specific configuration on your machine, unless you can reproduce it across many different machines).