I showed in a previous post that Safari is better than Firefox, on Windows, at handling pictures. Besides managing color profiles, Safari also offers a much better resizing algorithm. The pictures below show one of my photographs resized by Firefox 2 and Safari 3 on Windows. The original picture is 3620 x 2440 pixels large and I created a simple page that resizes this picture to 500 x 337 and 240 x 162 pixels. Move your mouse over each picture to see the difference between the two browsers.
You also look at a side by side comparison. Besides the jaggy edges, you can see banding in the sky, in the top right corner.
P.S: Telling me that the width and height attributes of an img tag are not the best way to resize an image would be pointless and would not make Firefox better on this particular case. I have to admit that I never use them though :)