In practice, an "open standard" that has only been suitably implemented by just one party really isn't all that open.
Now, maybe there's a greater chance that the situation will change, and other suitable implementations will eventually arise. But until that happens, an open standard with one usable implementation really isn't different from a closed standard at all.
I would counter that and say, "An 'Open Standard' created and then implemented by 1 party is no different than a closed standard".
However, Khronos[1], the entity that makes opengl, also made the webgl standard. The fact that mozilla hasn't done as good of a job implementing it as google has doesn't magically negate the fact that it is still an open standard (that mozilla has actually implemented, just poorly).
In fact, this uses threejs[1] under the hood. It is just that the version of Firefox someone is using sucks in comparison to Chrome for the three.js stuff (likely WebGL).
[1] http://threejs.org/