If owners install the Ford Racing FR-3 lowering springs and matched damper kit the basic Fiesta IS ALLOWED to participate. I personally have done this to my Then new 11 Fiesta hatchback.
I have never felt that the Fiesta was unstable with this kit. And, i will also say that i have spun the car out oncourse during both wet and dry events. It never felt like it would flip over. This needed to be said! The Fiesta can be a fun starter car for learning to autox.