The files mozilla-mail.desktop and mozilla-compose.desktop have icon names that end with a gif extension while the icons are .png files! FYI
Marking WORKSFORME. They are gif files here: mozilla-compose-icon.gif: GIF image data, version 89a, 23 x 21 mozilla-mail-icon.gif: GIF image data, version 89a, 16 x 12 I seem to remember fixing this way back when, but I'm not sure when that was.