This app has worked for me quite some time. Out of the blue, the add-on no longer appears on Gmail. I have refresh Firefox, uninstalled, reinstalled the add-on to no avail. I am running Firefox 65.0.1.
One of the best extensions I have for Gmail. Hope it will be updated to work with the new Gmail interface now! I won't use the new Gmail without it.... and please do not allow Google to block this from Gmail that would really annoy me.