https://addons.mozilla.org/en-US/firefox/addon/add-https/reviews/917110/
As per the user:
Even after I install, the install button remains available, as if I never installed it at all....when I did because it's in my add-ons.
I also see no button to switch on or off. I think this add-on is just not going through as installed. I wouldn't know why though. Never seen it happen before.