@eckhard, the 737 we do fly cannot really, and does not neet to dump fuel for landing, and of course the captain allways has the final authority. Fire on board is a special case and requires immediate landing where any kind of landing, including off airport or ditching, has to be considered. During simulator training taking more than 15 minutes to land with a fire still burning will automaticly fail you and require retraining in an additional simulator session. I do not know if our airbus fleets have to follow the same rule, however on the 737 we have to use full factors except if an immediate landing is required.
When operating under non normal conditions we have to check a factored non normal configuration landing distance (20% factor) against full factored normal configuration landing distance and use the higher value. The captain can decide to not follow that rule of course, but has to defend his decision later on if he is still able to do so.