This issue is strange. I did check your site in firefox in couple of my machines and the icons are displaying fine here. Can you please check in some other machine and see whether the issue is still there. Nobody else have reported this issue so far, so I am guessing probably it is happening only in your machine. Thanks!
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.