I have not used Lightbeam for a while now.

I am not sure it does actually block anything, but it does generate some very interesting Views & I found it informative to look before and after using ublock origin something that does block things for you.

Also see

• Lightbeam's home site & mozilla'sa about page
  That has screenshots, graphics, information & linked articles relating to lightbeam use and privacy in general.
• uBlock Origin 1.9.16 https://addons.mozilla.org/firefox/addon/ublock-origin/
  It works as is out of the box, is effective, and seems to have alow impact on Firefox performance, and does not sell your data, and has simple or advanced customisation features. Of course alternative add-ons are available.
  • Home site & documentation:
    https://github.com/gorhill/uBlock#ublock-origin
  • Support Forum:
    https://discourse.mozilla-community.org/t/support-ublock-origin/6746

My last answer was incorrect. I have tried Lightbeam again and it does indeed allow me to select and block sites. A feature it does document, but I had not tried or had forgotten about

" The List view provides more options for drilling down into individual sites. 

Site preferences in the List view enable you to block sites from connecting to your Firefox browser, "