I've gone back to Sep 2017 with mozregression and this bug is present at least that far back, which means that its probably a bug in Flash. This rendering mode does seem to work with the nptest plugin.

STR:

1. Run Firefox with dom.ipc.plugins.asyncdrawing.enabled set to false and WebRender off.
2. Go to https://www.ultrasounds.com/ and permit the Flash app to play.

Expected:
The logo with the animated colors plays continuously.

Actual:
About 3 frames of the animation play, then it stops. It advances one frame on mouse clicks and, more strangely, mouse-overs of the browser's window chrome buttons (e.g. window close). There may be something related to windows messages going on.

This bug is low priority since async rendering is on by default.

Bug 1296400 convinced me to take another look at this. The page in comment 0 has the animation bug but many don't. Some good swf examples are here [1]. This is still a bug, but is even lower priority.

[1] http://www.flashkit.com/movies/Animations/

Adobe Flash is no longer supported.