A personal opinion. Mine is it shouldn't. :)
This problem of resizing or not resizing text would disappear if browsers implemented a proper zoom feature such as Opera has. Currently if you want some text to remain proportionate to its surroundings the only safe way is to use an image, which is to my mind a backward step.
You may be right about FF and NN being at the same speed. Some time ago I measured NN, but my assessment of FF was purely visual.