Hi Seul,
I don't know about how the threshold works in the FSL viewer, but you could threshold the volume for good before passing it to FSL with 3dcalc with the step function.
3dcalc -a CORR_DSET -b CORR_DSET'[THRESHOLD_LABEL]' -expr 'step(b-THR)*a' -prefix BRUTE_THRESHOLD
where THRESHOLD_LABEL is the label of the sub-brick with which you are thresholding in AFNI, and THR is the threshold value.
cheers
z