Mokonafan wrote:
I guess Firefox is buggy. lol
I don't know if it's accurate to call it "buggy". The problem arises whenever a one-pixel thick line is scaled down; how should a fraction of a pixel be displayed? (If you render a grid in GIMP and look at the layer's preview thumbnail in the Layers dialog, you will likely see similar missing lines).
One solution would be to reduce the amount of color of the scaled-down line* (e.g., a black line of width 0.5 becomes a one-pixel wide gray line), but that requires either an alpha channel on the display and/or compositing of the line with the background. This approach requires a lot more calculation (and on some display drivers is not possible) and so it is not uncommon to use a quicker method which sometimes eliminates the line altogether.
* In GIMP, scale a layer containing a grid of one-pixel wide lines and you will see what I mean.