Use Firefox > Help > Submit Feedback

The most likely reason you can't find it might be because you don't have your Menu Bar up in your toolbar. If you don't want to find it you can just use your keyboard while on the site you want reported. Just press:''''' Alt > H > F''''' '''''Alt '''''will bring up the Menu Bar. '''''H '''''will select the Help tab. '''''F '''''will take you to the Report Web Forgery page. Hope this helps. :)