What is the difference between "Mobile-first Responsive" and "Twitter Bootstrap" on Initializr?
Bootstrap is mobile-first. Mobile first basically means you design for the mobile user in mind first, and then adapt as you increase window size. So there isn't a difference between "mobile-first" and Bootstrap. The difference is between Responsive and Bootstrap because Bootstrap is a framework and Responsive is more or less a "dumbed-down" version - less bloat if you don't need a bunch of elements.