Why did pfSense fork from m0n0wall
m0n0wall is a great system, but with a different focus. With its focus towards embedded systems, m0n0wall's design makes it difficult to take advantage of the capabilities of faster hardware with more storage capacity. The one main area where this is apparent is additional functionality. pfSense has packaging functionality that allows nearly limitless expandability of functionality. m0n0wall's file system design, while perfect for embedded systems, makes similar functionality in m0n0wall virtually impossible. In addition pfSense features multiple WAN support, high availability, and much more.
There may be some situations where m0n0wall will be more suitable than pfSense, primarily with very limited hardware resources.
For more information, see this message from m0n0wall's founder, Manuel Kasper.