Which logo are you referring to? Have you installed any custom themes? You can check this by going to Add-ons and then clicking on Appearance

This is with firefox pinned to my taskbar (the one in the middle)

This may have been cause by somehow changing the Firefox installation directory or some other program trying to change the link.

If you click Start and then search for Firefox, can you see the proper logo for it? If so, start it from search and then right click the icon on your taskbar and go to "Pin this program to taskbar".

If that fails to work then you can always uninstall and re-install (heh).