I have found some nice firefox icons and I'd like to change my old icon for one new one.

I have logged in as a root and went to usr/lib/firefox/icons and pasted the new icon in, but after restart it doesnt appear. It just shows a red circle instead of the icon. How could I make firefox accept the new icon?

NOTE: I have just switched back to the root account and it works perfectly there, but when I wanna use it on my normal account it wont work. I have tried to copy the icon in the same way I copied it with the root account, but it wont let me do it. Is there a way to get root access on my normal account and copy the icon?