This is the same type of hole that allowed Safari and OSX to be "pwned" at the Pwn 2 Own competition. A user simply has to browse to a compromised site (say through an e-mail link) and the code can be executed. The new JIT compiler is part of TraceMonkey a new optimization for FireFox 3.5.
This is a great example of how security can be inadvertently compromised for in return for speed improvements.
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