You will need to contact proctoru support.

What happens when you try to install it? Have you looked for it on the add-ons site?

https://addons.mozilla.org/firefox/

I guess this is why the add-on is not offered on addons from Mozilla.

This extension requires permissions to:

   Access your data for all websites
   Clear recent browsing history, cookies, and related data
   Get data from the clipboard
   Input data to the clipboard
   Extend developer tools to access your data in open tabs
   Access your location
   Monitor extension usage and manage themes
   Display notifications to you
   Read and modify privacy settings
   Access browser tabs
   Access browser activity during navigation