It doesn't appear when a site is using a huge overlay (which is what I installed this for, to delete that element). You may need to use a stupid high zIndex or something.

For others having trouble installing, I did it on beta, after enabling and adding the collection where the app reboots, you then need to go into settings->addons and hit the little plus. This last step isn't mentioned on the mdn blogpost. Then it turns up in the trippledot->addons on every tab.