Fundementals of Engineering exam with Math degree (not ABET-engineering?)
According to this National Council of Examiners for Engineering and Surveying (NCEES) website:

The NCEES board of directors recently approved an amendment to the policy that governs the NCEES Engineering Standard to expand access to its credentials evaluations service. NCEES uses this standard to evaluate educational credentials of engineering licensure candidates with degrees from the following:


Engineering programs outside the United States 
U.S.-based programs in engineering, engineering technology, related science, or mathematics that are not accredited by the Engineering Accreditation Commission of ABET, when coupled with a master’s degree or doctorate from a program that is EAC/ABET-accredited at the undergraduate or graduate level


So, it seems that having an undergraduate degree in math is not, by itself, a showstopper for sitting for the FE exam, provided that you have a MS or PhD in engineering from an institution that is ABET-accredited, either at the undergraduate or graduate level. In this case, it sounds like you will be referred to the NCEES Credentials Evaluations service to have your background reviewed in order to determine if you are prepared well-enough to pursue engineering licensure.