FF 24 is the latest stable. FF's docs say they added this function in FF 4.0, which was several years ago. So it's a bit more complicated than you make it out to be.
EDIT: I thought FF24 wasn't working, turns out I was confused between my Linux desktop's and Mac laptop's FF versions. Sorry!
EDIT: I thought FF24 wasn't working, turns out I was confused between my Linux desktop's and Mac laptop's FF versions. Sorry!