This is quite likely an exception. Browser JavaScript engines are faster than Flash's ActionScript engine, and WebGL is older and more mature than Flash's 3D support.
Now when will we see a truly open standard for WebGL? The Khronos Group is dominated by industry heavyweights who are designing a non-open standard that benefits their interests. Soon we likely won't be able to implement competitive web browsers without proprietary code from graphics card manufacturers. I am very unhappy with the direction of this even if the web clearly needs better graphics support.
Membership in the Khronos Group is open to anyone who's willing to pay their fees (which includes Mozilla, Google, Opera and a number of universities, by the way - not just GPU vendors). The specifications they produce are publicly available at no cost. They even have a public mailing list so you can follow the discussions of the working group. How much more open do you want?
Maybe a better question: if that isn't what you consider "open", what is?
No it doesn't. Adobe did open-source their JIT and partnered with Mozilla for further development. But Mozilla could not make it fit Javascript well enough, and eventually their own JITs became faster, so they concentrated on them.
When wasn't it faster than Flash? Last time I used Away3D, I had trouble getting more than 5k triangles with a reasonable framerate, which is a bit ridiculous.
Admittedly, this was a couple years ago. Has something changed?
Yes, Flash got a hardware accelerated 3D API called Stage 3D. A performance comparison could be very interesting, if it had some actual data behind it.
Away3D 4.x uses the new Stage3D flash feature ("OpenGL wrapper") for hardware acceleration. "A few years ago" means you probably used the old Away3D 3.x software renderer.
