That's because the Pub 2003 html coding engine produces different copies of
your images from your original with the goal of providing the "best" image
depending on which browser you use. As you have discovered, that isn't
always the case.

I have tried to find the best workaround for this, but haven't settled on
the best. You definitely should go to tools > options > web tab and uncheck
"rely on vml..." and "allow png...".

It also seems that if you resize and optimize your images before you insert
them, and then make sure the images are at full size, this can sometimes
help. Select the image in your Pub doc > Format > Picture > Size tab and
make sure the Scale is set for 100% for both height and width.

And finally, if you import the images into your site, rather than
embed/insert them, this can also help as FireFox will display your original
image, not a copy.

DavidF