One of my favorite themes for Google Chrome is "Zen Spring". Google only made it for Chrome, but I also wanted it for Firefox! Since I couldn't find a pre-made theme, I went ahead and made one.
My first attempt was using Firefox's online theme maker. That one places images on the right, so I just fed it the original Zen Garden image.
For my second attempt, I tried to clean up the source image and then put together a proper theme using a manifest file (so that the tree image was on the left, like in the original theme).
You can click the preview images to go to the Firefox site to download them.