The difference here is that we wanted to implement a multiprocess architecture. We don't want to implement (P)NaCl, since we don't think they're good for the web. We have no qualms with integrating code from other sources, we've done a lot of it. (Google Breakpad, WebRTC, numerous image and video decoding libraries, Freetype, libffi, ANGLE, the list goes on...)
As a further observation on this, Robert O'Callahan has a blog post [1] explaining Mozilla's stance on (not) implementing bad-for-the-web features; as an example to show this stance is long-lived, he mentions that engineers at Netscape/Mozilla implemented (in 1999) ActiveX support for Gecko, but kept it disabled by default on purpose despite a possible market share impact.
The reason why game devs use Scaleform is because of the great vector graphics authoring tool. WebGL is just a gl context in javascript and gives you nothing more than before.
No, WebXXX isn't always the natural next step HN.
