can not install Gnome extensions
Solution 1:
Regarding Firefox
After visiting the Gnome Extension page you have to allow the plugin execution - otherwise your Gnome Session will not be recognized.
Therefor click on the plugin icon to the left of the Firefox addressbar and allow the execution.
Solution 2:
Install chrome-gnome-shell depending your distro and run one time:
/usr/bin/python3 /usr/bin/chrome-gnome-shell --gapplication-service
After this the addon from chrome will detect the native plugin.