URL extensions.gnome.org for managing GNOME Extensions not working in any browser

You need to install chrome-gnome-shell package so that your browser can detect your running GNOME Shell version. To install it run

sudo apt install chrome-gnome-shell

Note: Installing GNOME Shell extensions doesn't work with the snap versions of the browser applications. Also since the 19.10 release, Ubuntu offers only the snap version of Chromium, the apt version of Chromium is not available in the standard repos.

I also followed all the instructions and tips and still couldn't get it to work with Chromium in Ubuntu 20.04 as Chromium is offered only as a snap package since Ubuntu 19.10.

But I did get it working with Brave browser, with the same Chrome extension. So one solution would be switch to Brave (it is also based Chromium, so not a big difference).