Usually this icon is added automatically the first time you install Adblock Plus, but maybe it was removed somehow afterwards. In Firefox you can customize the toolbar, simply right-click on the toolbar and choose "Customize". A window with icons will open, Adblock Plus icon should be one of them.
Drag it with the mouse into the toolbar and place it where you want it to be. You can also reorder the toolbar icons by dragging them while the customization window is open. More.