Turning off TI makes the problem go away... I also see the problem in nightlies.
Created attachment 588291 [details] [diff] [review] patch Regalloc bug when doing a branch based on values known to be floating point numbers. We didn't take care to ensure that sync code did not clobber the registers that were about to be compared, so the comparison could end up using incorrect values in the lhs/rhs. I like the website, I've seen a fair number of those critters while diving here in california.
This is a regression in FF9, and we're not seeing any dupes, so I'm untracking for 11/12/13. This can ride the trains.