I checked your blog on my firefox installation and looks normal (do not know if you fixed it already). I think you have nothing to worry and have some issue local to your desktop. The reason I say this is because I went to http://www.browserstack.com/ and generated screenshots for your blog for firefox installations of various versions and for various flavors of operating systems and every single one of them rendered the icons fine. You can check that out yourself if you are still facing this issue on your machine.