Disabling JIT will roll back your FF 3.5 speed to previous version (3.0).It will decrease JS performance.Beside, it will also block updating issue to next version FF (3.5.1 ;3.6 etc)
IMO, only recommended as temporary security measure.
I found useful article regarding security measurement in browsers at US-Cert
at last, for those concern about security ,run FF in safe mode.