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JägerMonkey
JägerMonkey is an additional optimization to the engine in Firefox 4. It improves speed in cases where TraceMonkey optimizations fail to improve performance.[7][8]
The most publicized optimization in JägerMonkey is "method-based JIT" (that compiles code one method at a time) using inline threading (see [9]) and implements PICs, but there were multiple other optimizations implemented for Firefox 4.[10] To implement method JIT, Mozilla reused the assembler from Nitro, WebKit's JavaScript engine.[8]
The difference between TraceMonkey and JägerMonkey JIT techniques and the need for both was explained in a hacks.mozilla.org article. A more in-depth explanation of the technical details was provided by Chris Leary, one of SpiderMonkey developers, in a blog post. More technical information can be found in other developer's blogs: dvander, dmandelin.