is an alternative icon for firefox , in black and white and other in black and orange

installation =
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 , ....
[scalable/apps]
Size=512
Context=Applications
Type=Threshold
MinSize=24
MaxSize=1024

or you can convert de svg to png and put in right places, but you should convert to many sizes !!

regards