var prefs = Components.classes['@mozilla.org/preferences-service;1']
                    .getService(Components.interfaces.nsIPrefService).getBranch('layout.css.');

switch (prefs.getCharPref('devPixelsPerPx')) {
  case ('-1'):
  case ('-1.0'):
    prefs.setCharPref('devPixelsPerPx', '1');
    break;
  default:
    prefs.setCharPref('devPixelsPerPx', '-1.0');
}