Sometimes, when an image is resized, there's a missing line at the
bottom. I've attached:

  * A testcase with 2 images: on the left, the image resized by
    Iceweasel (missing line at the bottom); on the right, the
    image already resized with ImageMagick (correct).

  * A screenshot showing what I obtain.

I could reproduce the problem with a Firefox Nightly from last year,
but when I upgraded to the latest nightly, the problem was no longer
there. So, it could be an upstream bug that has now been fixed.