I'm pretty sure I have the new FairRoot, as some functions have be changed (SetInputFile has moved to FairFileSource, SetEventMeanTime to FairSource [I think]).
I now see why I was offered the choice for ROOT 6 (which also started my other thread on the compilation errors); I was using the instructions from the Step-by-step installation on https://github.com/FairRootGroup/FairRoot, which tells you to use