http://www.aqua-soft.org/forum/topic/28029-help-change-firefox-taskbar-icon/
"There's another way to do this;
1. Go to the folder you installed Firefox in (e.g. C:Program FilesMozilla Firefox) and then go to the subfolder chrome.
2. While in chrome folder, create a new subfolder called ''icons'', then go to that folder and create yet another subfolder called ''default''. The full path to this folder could be C:Program FilesMozilla Firefoxchromeiconsdefault.
3. Choose the icon you want to use and place it in the ''default'' folder and rename the icon to ''main-window', e.g. main-window.ico on Windows and main-window.xpm on Linux.
In addition to the main window, you can also change the icon on the Bookmark Manager and JavaScript Console. The icon names are ''bookmark-window''.[ext] and ''jsconsoleWindow''.[ext], respectively.
After this is done, restart Firefox and you should be able to see your chosen icon in the title of the browser windows."
