Hi,
Try setting the shortcut to "read only" after assigning your custom icon.
Or, if these are internet shortcuts, IE may be changing the icons when checking for a Favicon. Creating a shortcut to FireFox which includes the URL will make the shortcut a LNK file and IE should leave it alone. A side effect is that each shortcut will open in a new window.
Find "Firefox.exe" (probably in "C:\Program Files\Mozilla Firefox\firefox.exe")
Right-click and drag firefox.exe to the desktop and choose "Create
shortcut." Right-click that shortcut> Properties> Target, modify the Target to include the URL.
Example:
"C:\Program Files\Mozilla Firefox\firefox.exe"
becomes
"C:\Program Files\Mozilla Firefox\firefox.exe" www.yahoo.com
Note a space between EXE" and www
Rename the shortcut if desired.
Apply the custom icon.
Since it's no longer an Internet Shortcut, it's an .lnk (.LNK) file, IE
should leave the icon alone.
Hope this helps