How do I get AppCenter to stop trying to download/install Thunderbird? I just installed elementary 5.1 using VMWare Fusion 11.5.1 on a Macbook Pro running Mojave 10.14.6 I went to AppCenter to download/install Thunderbird. After selecting Thunderbird, AppCenter now says waiting for authentication but there is no authentication screen. AppCenter is showing a spinning icon next to the search window.
Try install Thunderbird with command line
sudo apt install thunderbird
maybe that will fix your problem