Cannot install issue

[elekt]

Hello!
Firefox 50.1.0, Ubuntu 16.04
When I try to install from the https://addons.mozilla.org/en-US/firefox/addon/cupcakebridge/?src=ss
In the end comes the pop up that it can only install if I restart firefox. If I do click restart it does, but doesnt seem that it installed cupcake (no icon on the toolbar)
However, it does list in the "Add ons" of firefox, but without an icon.
Probably I'm missing something, but Id like to see an indicator if my browser is a proxy.