Hello,
I'm running Firefox 56.0.1. If I press a link Add to Firefox on the Mozilla addons site several alerts appear reporting as the addon download progresses. Orca reports these as warnings. These are not focusable so are just informative.
When the download is finished there is a warning saying something like Download is finished. This site attempts to install an addon. I can press alt+d to move to the addressbar. When the addressbar is focused I can press shift+tab key to jump to the button saying something like opens addons manager to install the addon. I can press space on this button and a dialog will pop up where I can navigate with tab or shift+tab to buttons such as Add or Cancel.
I am not sure which version this dialog has been made accessible. So if it is not working like this in your version you will have to resort to flat review.
