This is a bit of a nuisance, after a fresh install of Ubuntu Gnome, I was not able to install extensions from extensions.gnome.org.
Firefox asked me if I’d like to install the extension but after a Firefox restart I still wasn’t able to install any plugins.
To be precise; this message was shown:
Although GNOME Shell integration extension is running, native host connector is not detected. Refer documentation for instructions about installing connector.
The solution was to install the chrome-gnome-shell package;
sudo apt-get install chrome-gnome-shell
This fixes the message from both chrome and firefox.