On Firefox4, there are three JIT options:

javascript.options.jitprofiling.*
Tuning JIT compiler heuristics; Profile to decide using tracegit or using methodjit.
javascript.options.methodjit.*
JägerMonkey; Baseline JIT compiler.
javascript.options.tracejit.*
TraceMonkey; native-code compilation.
javascript.options.*.content
Whether it apply to webpages or not
javascript.options.*.chrome
Whether it apply to firefox's interface (XUL) or not

Note: javascript.options.jit.* are replaced with javascript.options.tracejit.*.