I have included with this post a picture of the official Firefox logo. If your icon looks like that, then it is correct.

Try to rebuild the Windows icon cache.

• Open the Task Manager (Shift+Ctrl+ESC)
• In the Process tab, right-click on the Explorer.exe process and select End Process.
• Open the file picker via "File > New Task (Run)" and click the Browse button.
• Type or Paste %USERPROFILE%\AppData\Local in the File name field (AppData is a hidden folder).
• Select the file IconCache.db and use Delete in the right-click context menu to delete the file.
• After the IconCache.db has been deleted, start a new explorer.exe process via "File > New Task" to get the desktop and Taskbar back.

Verify in Windows Explorer that you can see the IconCache.db file before doing the above mentioned steps because the file IconCache.db may be a hidden file. See http://kb.mozillazine.org/Show_hidden_files_and_folders