Errors with this are very easy to see, and would be unacceptable even for snapshots or other simple photos.
Steps to reproduce seem to be from a different bug
They are similar to the other bug I reported (543015) but not the same. They are two related but different bugs.
Which element's coordinates do you mean as percentages exactly. The linked image has width="512" height="512" which are not percentages. What artifacts (add a screenshot using the Add an Attachment button above) and exactly what steps to see these artifacts.
Created attachment 424254 [details] Image above has <image x="50%" y="50%" and image below has <image x="256px y="256px". Zoomed in at 800% you can easily see the difference.
You're not displaying the images in their original size in both cases. You can use image-rendering: -moz-crisp-edges to avoid this if you want to.
You are scaling one of the images and therefore getting bicubic interpolation. Use image-rendering to avoid this.
Or alternatively fix your viewBox so you aren't scaling the images.
I don't think the problem has much to do with scaling. I opened up http://www.go2mk.com/temporary/Firefox_SVG_Bug/pattern-test-before.svg then used firebug to modify the viewBox to 0 0 512 512, so there's no scaling being done (the PNG image is 512 by 512), not even at a decimal level (which is what I had before), and the problem still occurs. However, I did notice the problem has to do with the EXACT location of the image on the screen. As I was resizing firebug, each pixel up or down would break or fix the image (after modifying the viewBox to 0 0 512 512). I am guessing if the container size becomes and odd number then 50% would produce a decimal, which breaks the horizontal lines. Do you see what I am describing now?
We can't draw something at x.5 pixels so we have to scale it to get the width right.