First, there is no Flash 6 or FM.
Flash MX is the next step up from Flash 5 (i.e. 1,2,3,4,5,MX...??)
Hope that helps, without going into alot of detail.
There is nothing that is Flash FM, that might be cause of your confusion:
The lowdown: There is not a release called "Flash 6."
You would think there would be, considering the previous version was 5.
However, I believe Macromedia's intention with MX is to go beyond what anyone expected.
Alot of people have refered to it as Flash "Macromedia eXpanded," as with this new release there are so many things that are new, and beneficial in ways that Flash 5 couldn't be.