My FF doesn't show the Ad Block icon either because I don't show the "Add On" toolbar. (View -> Toolbars -> Add-On Bar). I don't get ads though.
If it's installed, and you're not getting popups, I wouldn't worry about it. Just surf away to your heart's content.
Tools -> Add-Ons should show the part in red:
[ATTACH=CONFIG]434680[/ATTACH]