Click the button on the right to install the addon. The listed permissions are required to block websites. No data collected is ever sent off your computer.
If nothing happens when you click install try copying and pasting about:addons in your address bar to make sure the addon is enabled.
If you get a connection error right click the install button and select "Save Link As..." Then, browse to the .xpi file and open it with Firefox.
If you get a file corruption error please update Firefox to at least version 109 or install the older extension from here.
After installing the addon, you'll be guided on how to enable the extra permissions required.