The biggest addition in Firefox 18 is  Mozilla’s new JavaScript JIT compiler called IonMonkey. This promises a  noticeable performance  improvement whenever Firefox is displaying Web apps, games, and other JavaScript-heavy pages.

IonMonkey provides a brand new architecture that allows for optimizations to be introduced into JavaScript JIT compilation. It differs from previous compilers by including an intermediate representation (IR) and has three steps:

1. Translate JavaScript to an intermediate representation (IR).
2. Run various algorithms to optimize the IR.
3. Translate the final IR to machine code.

The optimizations include:

• Loop-Invariant Code Motion (LICM), or moving instructions outside of loops when possible.
• Sparse Global Value Numbering (GVN), a powerful form of redundant code elimination.
• Linear Scan Register Allocation (LSRA), the register allocation scheme used in the HotSpot JVM (and until recently, LLVM).
• Dead Code Elimination (DCE), removing unused instructions.
• Range Analysis; eliminating bounds checks

The code generation works for x86 both 32bit and 64 bit and for ARM. 

The new design is similar to the techniques used in Chrome's Crankshaft JavaScript engine and should make the two similar in JavaScript performance.

Other new features in Firefox 18 are:

• Retina Display Support for Mac users to make Firefox even sharper when watching movies, playing games and browsing the Web.
  
  
• A facility to disable insecure content on HTTPS secure websites to maintain the privacy of your communication with the website. You can enable the feature in about:config.
  
  
• Support for standard W3C touch events in addition to MozTouch events.