I think for MVP sake it makes sense to make it work for people with modern browsers first. If it generates a passionate enough user base there, then they can put in the resources to make it compatible.

Sorry, webkit has won. Firefox and IE are slow, antiquated technologies. For an MVP product, it's just a waste of time to try to put in the dev effort to make things work across every browser. People who happen to be early adopters tend to be using Chrome exclusively (almost none use IE).

(i have no affiliation with Hoodsup)