I've got separate installations of Firefox from version 3.6 all the way up to version 10 (so far!). These are for testing purposes. I've managed to change the icons from versions 3.6 to 9 (7 in total!) to a custom icon with the version number so when I have multiple versions open I can easily tell which is which. I can't for the life of me remember how to do this though! I've just installed version 10 and I can't change its icon without pinning it to the taskbar. None of the other custom icons are pinned and they always override the default Firefox icon when I launch it.
I seem to remember that I need to either add an icon to a particular folder or delete the default one that gets created but I can't remember which or where the folder is (if I'm right).
In C:\Users{My User Name}\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar I have only the icons I want pinned.
I've tried pinning the program, changing the icon and the name of the shortcut and then unpinning it but it always reverts back to the standard icon.
I have custom icons for all versions on my desktop and in my Start menu.
OK, since nothing I did worked I tried reinstalling Firefox 10. I let it create its own icons on the desktop and start menu, then I changed the icon from the taskbar (right-click program icon on taskbar, right-click on program name in pop-up menu, select properties, change icon) on its first run. Now it's not pinned but I have a custom icon for it just like the others! The answer seems to be to let the program create its own shortcuts rather than adding a link manually, then edit the existing shortcuts.