We have had  a fire door installed in our building but the floor is not level. The door does not have to have smoke control but  there is a 10mm gap under the door but the manufacturer states the fire door should have a 6mm gap. Could we reduce the gap with a surfaced mounted drop down seal? 

Before you do anything, talk to the technical guys at the door manufacturer.  Often they have more than one form of evidence of performance so it's sometimes (not always) the case that a 10mm threshold gap would be permissable.