Okay, changed it back to confirmed, since you requested that and confirmed its existence at the same time. It is up to someone else to decide if the real problem lies with thunderbird or gnome-software. Thanks!
Maybe my "fix" worked becase I installed thunderbird with apt first and then cleaned the cache, so it's not really a fix. Thunderbird did later appear in gnome-software, but did so in the installed state, not ready to install as it would be desired.
Okay, changed it back to confirmed, since you requested that and confirmed its existence at the same time. It is up to someone else to decide if the real problem lies with thunderbird or gnome-software. Thanks!
Maybe my "fix" worked becase I installed thunderbird with apt first and then cleaned the cache, so it's not really a fix. Thunderbird did later appear in gnome-software, but did so in the installed state, not ready to install as it would be desired.