is an alternative icon for firefox , in black and white and other in black and orange
the most simple is right click in the icon and change it.
rename the icon to firefox.svg
if you are using a pack of icons svg then put it in nameicons/scalable/apps . if you don't , then make the directory and put the svg and add the dir to nameicons/index.theme =
Directories=scalable/apps , ....
or you can convert de svg to png and put in right places, but you should convert to many sizes !!