When I want to install an add-on I tap on "Tools" -> "Add-ons" -> "Browse all Firefox Add-ons" which opens a new tab for https://addons.mozilla.org/de/android where I search for an add-on I want and then tap on it. Now I tap on "+ Add to Firefox" and it says "Blocked add-on Firefox prevented this site (addons.mozilla.org) from asking you to install software on your device" that shouldn't happen in the first place. I then can choose "dismiss" or "allow" so I tap "allow " and with some add-ons it then says "downloading addon" -> "Installing Add-on (name of the add-on)|cancel|install" which is fine, but for other add-ons e.g. https://addons.mozilla.org/de/android/addon/bookmark-all-tabs/ it first says "downloading addon" and then "Blocked add-on|firefox has prevented addons.mozilla.org from installing an unverified add-on android|learn more". So I tap on "learn more" which opens https://support.mozilla.org/1/mobile/49.0/android/de/unsigned-addons where I am told that "All add-ons hosted on addons.mozilla.org are signed before they are listed." which isn't helpful.