While Firefox is opened and pinned on your taskbar right click on the taskbar icon, then right click on Mozilla Firefox (right above "Unpin this program from taskbar"), then click on properties and select "Change icon..." from the Shortcut tab. Navigate to the Firefox.exe in "C:\Program Files\Mozilla Firefox\firefox.exe" and select the Firefox icon file, and select open. It should now show all the available icons you can choose from. Select the one you want, and click "OK". Now unpin Firefox from the taskbar, and the icon should show. Re-pinning should pin it back to the taskbar with the icon.