These are my modifications of Firefox's classic.jar to use Nuvola icons instead of.. er.. whatever the hell they call the ones they have to start with.
There are two versions of classic.jar - one with 16x16 small icons, and one with 22x22 small icons, which looks better to me. As this is a craptastic effort, there are no full sized nuvola icons in the pack, although if there's enough interest I might do those.
To use, go to [firefox installation directory]/chrome/ and replace classic.jar with one of these (after backing up the original if you are wise).
Should work on Ff 0.9x, don't know about other versions. Enjoy!