I'm trying to install some GNOME Shell Extensions on Ubuntu 16.04 (64bit) from the web site extensions.gnome.org. I'm using Gnome 3.18 with LightDM.
I verified that Firefox has the Gnome Shell Integration plugin enabled, and the extension page loads with the on/off slider correctly displayed.
When I switch the slider to "on", I accept the dialog about installation but then nothing happens. After reloading the page, the slider is still "off". Thus the extension is actually NOT installed and the directory .local/share/gnome-shell/extensions/ remains unchanged.
I read older discussions whereas someone were suggesting to reset GNOME settings to default, as discussed here: How do I reset GNOME to the defaults?, but that did not solve the issue.