Firefox does not use points as font size unit. It uses pixels. This is why you need to specify 16 (px) fonts in Firefox to get roughly the 12 (pt) size on a typical ~ 100 dpi screen (and yet another value on your system, that you need to change for every freaking encoding because someone decided having encoding-specific sizes was smart)
Of course this is a usability disaster. I've always felt exposing pixel sizes this way when the physical size of a pixel can vary greatly from one screen to another was a way to break so utterly text size adjustment no one could tell what dev made the biggest mistake anymore.
(People will raise the "angle of view" argument. Since no one can measure "angle of view" automatically this is just a way to refuse fixing the problems. Physical sizes can be autodetected and they're good enough for many cases)