Mozilla's David Mandelin sheds some light on the latter in a recent blog post. Dubbed IonMonkey, the new JIT compiler is currently in the design stages and should be significantly faster than the existing JägerMonkey, with improvements in how it handles function inlining, dead-code elimination, and a few other stuff I wont pretend to understand – details here. Basically it's more efficient at translating the code a human wrote into the instructions a computer understands.
Mozilla also announced enhancements for the Gecko layout engine in a separate post by developer Joe Drew. Project Azure, as it is known, focuses on the creation of a new 2D graphics API for Firefox that's "significantly closer to Direct2D" as well as new 3D backends for drawing content in an accelerated way to OpenGL, Direct3D 9, and Direct3D 10. The intent with these 3D backends is to enable and improve hardware acceleration across platforms and devices.