I see what's going on here. Mozilla's trace compilation approach has a lot of startup overhead but produces extremely fast optimized code for inner loops. Therefore, their benchmark includes compute-intensive tests with long-running inner loops to amortize out the costs of JIT compilation and focus on the raw speed of the generated code, where they beat Chrome.

(Note that I'm not saying this is unfair or biased at all. This is a good set of benchmarks to have.)