bad to start 1.6 and plan to go to FF later ?

It has been advanced without explanation that it would not be a good idea to start now with a 1.6 and plan to move to a FF in three to four years. But it's really what I want to do. So why is that bad? (Yes I know a 1.6 won't do on a FF, but beyond that...)

Thanks for any help on this and just plain thanks for the valuable resource this site is.