I don't know about the latest FF stable, but the nightly builds of FF9 are no joke. It blows the latest Chrome/Chromium out of the water on the Sunspider Javascript test.
TEST COMPARISON FROM TO DETAILS
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** TOTAL **: *1.65x as slow* 309.9ms +/- 2.0% 509.8ms +/- 5.7% significant
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3d: *1.54x as slow* 48.4ms +/- 3.4% 74.3ms +/- 4.2% significant
cube: *1.38x as slow* 19.3ms +/- 3.9% 26.6ms +/- 6.7% significant
morph: *2.28x as slow* 9.6ms +/- 3.8% 21.9ms +/- 8.9% significant
raytrace: *1.32x as slow* 19.5ms +/- 5.0% 25.8ms +/- 3.6% significant
access: *1.77x as slow* 32.8ms +/- 3.8% 58.0ms +/- 10.8% significant
binary-trees: - 4.8ms +/- 9.4% 4.2ms +/- 22.4%
fannkuch: *1.73x as slow* 13.5ms +/- 3.7% 23.3ms +/- 12.3% significant
nbody: *2.41x as slow* 8.0ms +/- 4.2% 19.3ms +/- 23.2% significant
nsieve: *1.72x as slow* 6.5ms +/- 7.8% 11.2ms +/- 15.6% significant
bitops: *2.24x as slow* 21.8ms +/- 5.3% 48.9ms +/- 12.7% significant
3bit-bits-in-byte: *6.33x as slow* 1.2ms +/- 25.1% 7.6ms +/- 14.8% significant
bits-in-byte: *1.57x as slow* 8.8ms +/- 5.1% 13.8ms +/- 16.2% significant
bitwise-and: *2.67x as slow* 5.7ms +/- 11.9% 15.2ms +/- 13.3% significant
nsieve-bits: *2.02x as slow* 6.1ms +/- 10.3% 12.3ms +/- 14.2% significant
controlflow: *1.84x as slow* 3.7ms +/- 9.3% 6.8ms +/- 8.3% significant
recursive: *1.84x as slow* 3.7ms +/- 9.3% 6.8ms +/- 8.3% significant
crypto: *1.32x as slow* 25.9ms +/- 6.6% 34.3ms +/- 9.5% significant
aes: *1.151x as slow* 14.6ms +/- 11.8% 16.8ms +/- 8.0% significant
md5: *1.35x as slow* 6.6ms +/- 7.6% 8.9ms +/- 12.8% significant
sha1: *1.83x as slow* 4.7ms +/- 7.3% 8.6ms +/- 29.1% significant
date: *2.10x as slow* 40.9ms +/- 4.3% 86.0ms +/- 6.7% significant
format-tofte: *1.25x as slow* 23.1ms +/- 4.5% 28.8ms +/- 10.4% significant
format-xparb: *3.21x as slow* 17.8ms +/- 5.3% 57.2ms +/- 7.1% significant
math: *1.58x as slow* 32.1ms +/- 2.7% 50.8ms +/- 14.0% significant
cordic: *2.68x as slow* 5.3ms +/- 6.5% 14.2ms +/- 27.7% significant
partial-sums: *1.147x as slow* 20.4ms +/- 3.8% 23.4ms +/- 10.5% significant
spectral-norm: *2.06x as slow* 6.4ms +/- 5.8% 13.2ms +/- 13.2% significant
regexp: 1.36x as fast 20.5ms +/- 4.7% 15.1ms +/- 4.7% significant
dna: 1.36x as fast 20.5ms +/- 4.7% 15.1ms +/- 4.7% significant
string: *1.62x as slow* 83.8ms +/- 1.8% 135.6ms +/- 6.7% significant
base64: *1.125x as slow* 8.8ms +/- 8.4% 9.9ms +/- 11.0% significant
fasta: *1.92x as slow* 11.6ms +/- 4.3% 22.3ms +/- 8.3% significant
tagcloud: *1.23x as slow* 24.8ms +/- 3.5% 30.4ms +/- 11.6% significant
unpack-code: *1.53x as slow* 28.6ms +/- 2.9% 43.7ms +/- 6.9% significant
validate-input: *2.93x as slow* 10.0ms +/- 5.8% 29.3ms +/- 9.6% significant