Why is the melting point of KBr higher than that of CsCl?
It may be helpful considering molecular weight for say $\ce{KBr}$ vs $\ce{KCl}$ or $\ce{CsCl}$ vs $\ce{CsBr}$, and actually melting point would go down with increasing molecular weight, but it actually has nothing to do with molecular weight despite the trend. It would be more helpful to look at lattice energy, electronegativity, electron affinity, and atomic radii, but its a little more complicated to compare a $\ce{Cs}$ salt of a halide with a $\ce{K}$ salt of a different halide as you've changed more factors.  Although chlorine has a higher electronegativity and smaller atomic radius than bromine, caesium has an even larger atomic radius than potassium (relative to the size difference between chlorine and bromine) as well as a lower electronegativity than potassium.  All of these factors will affect the lattice energy and therefore the melting points.  But, like I said, when comparing two similar salts, make sure one of the elements stays constant.  Its much more tricky to compare $\ce{KBr}$ with $\ce{CsCl}$ than it is to compare ($\ce{KBr}$ with $\ce{KCl}$) or ($\ce{CsBr}$ with $\ce{CsCl}$), or even ($\ce{KBr}$ with $\ce{CsBr}$) or ($\ce{KCl}$ with $\ce{CsCl}$).  When comparing $\ce{KBr}$ with $\ce{CsCl}$, you must determine what will affect melting point more: the ($\ce{Cs}$ vs $\ce{K}$) difference or the ($\ce{Cl}$ vs $\ce{Br}$) difference.