I assume your referring to either the desktop icon or the panel icon. For the panel icon, right click on the icon, and select "properties" from the menu. A dialog box will appear. On the right side there will be "Name," "Description," and then a little box with the current icon, followed by a field, followed by an icon of a folder. Click on that icon of the folder, and then navigate to /usr/share/icons. Since I'm not sure which exact icon you want, you'll need to sift through there to find the right Firefox icon. For the desktop icon, right click on the icon, and select "Edit Launcher." The process for changing this one is almost exactly the same as for the panel icon.