GNOME extensions add various functions and tweaks to the GNOME Shell which is the default Desktop Environment used by Ubuntu 17.10 and later.
Extensions can be found on the https://extensions.gnome.org/ website.
1 Using Google Chrome/Chromium
Open the GNOME Launcher, click on Terminal and type in the below command:
sudo apt install chrome-gnome-tool
2 Using Firefox
3 Installing Extensions
After completing the above steps for your browser, explore the extensive library of extensions.
When you find one that you like, simply click the On/Off switch to turn that extension on.