add min--moz-device-pixel-ratio to make @2x variants work in firefox
add min--moz-device-pixel-ratio to make @2x variants work in firefox

ecmartinez/nib,seleckis/nib-pushup,F4-Group/nib,seleckis/nib-pushup,F4-Group/nib,JulianWielga/stylus-less,tj/nib,rstacruz/nib,ilovezy/nib,ecmartinez/nib,ilovezy/nib,tj/nib