A pure CSS lightbox effect is nothing more than a clever use of the CSS3 :target pseudo-class. Oddly enough, standard compliant browsers diverge when it comes to apply CSS3 transitions to this particular CSS state. Chrome runs all the transitions smoothly, while Firefox simply applies all the styles at once and without the correct timings previously set on the elements involved. Anyway, I hope you find it useful.
Demo
Live demo (with code)