We investigate the equilibrium geometry and electronic structure of nanowires using { ab initio} density functional calculations. The skeleton of these unusually stable nanowires consists of rigid, functionalized Mo octahedra, connected by flexible, bistable sulfur bridges. This structural flexibility translates into a capability to stretch up to 20% at almost no energy cost. The nanowires change from conductors to narrow-gap magnetic semiconductors in one of their structural isomers.