The latest Firefox comes with some new technology under the hood
Another big addition is full support for plugin-free video calls via WebRTC. Services part of the Social API can now be managed via the Add-on Manager.
The CSS3 Flexbox model, the final specification, is supported by default as well. There are several other developer updates.
This subset is limited by human standards, i.e. it's hard to write code for Asm.js by hand, though by no means impossible, people still write assembly code after all.
To prove this, Epic took its Unreal Engine 3 and ported it to the web, with spectacular results. Coincidentally, Firefox 22 also ships with asynchronous canvas updates while using WebGL which should provide a performance boost in 3D applications.
The other big addition to Firefox is full support for WebRTC. While there's still work to be done, Firefox now supports out of the box the three main components of the communication technology. You can read more about WebRTC in Firefox 22 here.