I just finished a clean install of Avast and opened Firefox for the first time since the Avast Free install.
As you can see, there shouldn’t be a problem adding AOS to Firefox
http://screencast-o-matic.com/screenshots/u/Lh/1499259454438-73285.png
http://screencast-o-matic.com/screenshots/u/Lh/1499259239498-16035.png
http://screencast-o-matic.com/screenshots/u/Lh/1499259310683-35726.png
http://screencast-o-matic.com/screenshots/u/Lh/1499259531727-31390.png
What other add-ons do you have installed in FF ??? Maybe one of them is iterfering with the addition ???