Unable to download updates from extensions.gnome.org
I was having same problem, no Internet issue was there. I did following -
Installed gnome-shell-extensions again
sudo add-apt-repository ppa:gnome-shell-extensions/ppa
sudo apt-get update
sudo apt-get install gnome-shell-extensions
I reinstalled gnome shell extension in firefox, and after some time, it started working. After that Ubuntu Software Manager showed no such error.