I noticed today that the official Firefox icon is still using the old 128×128 resolution, rather than the new 512×512 icon size that is the standard in OS X Leopard and I think Windows Vista too. Since the Firefox icon is a vector image, it is (in theory) infinitely scalable, so it should be a breeze to make a larger one. In light of that fact, I made a larger icon, which you can download here.
For comparison, here’s the new icon compared to the old one, at 512×512 resolution (click for full resolution):
To use the new icon, you can get a program like Candybar, or simply ctrl+click on firefox (in the /Applications folder), choose Show Package Contents, and replace the icon in the Resources folder.