TB68 is EOL. As of TB78 Lightning has been integrated into Thunderbird. Hence there's no need for a separate Lightning extension anymore. If you're using a Thunderbird version provided by your distribution you should ask in their support forum.

Ok,I installed TB 78. But it cannot be started. Don't know, why. I will ask that in an Ubuntu forum.

But why has TB 68 a calendar tab on the old workstation, and on the new one the same version doesn't?

Ok,I installed TB 78. But it cannot be started.

If this is supposed to be a problem description, you'd to be a little more specific.

But why has TB 68 a calendar tab on the old workstation, and on the new one the same version doesn't?

I'm wondering how you know when the new version doesn't even start.

It would also help to know which version you installed - the vanilla version from https://www.thunderbird.net/ or the version provided by your distribution? In the latter case you should ask in their support forum.

Ok,I installed TB 78. But it cannot be started.

If this is supposed to be a problem description, you'd to be a little more specific.

But why has TB 68 a calendar tab on the old workstation, and on the new one the same version doesn't?

I'm wondering how you know when the new version doesn't even start.

Before I installed TB 78, there was installed TB 68, and this one didn't have a calendar tab. The two icons in the top right corner to display it and the other one (tasks or so) weren't available too.

It would also help to know which version you installed - the vanilla version from https://www.thunderbird.net/ or the version provided by your distribution? In the latter case you should ask in their support forum.

The software app of Ubuntu offers both TB 68 and TB 78 (at different places). So I used it to uninstall T 68 and to install TB 78. The software app displays a button to start the installed version (now TB 78). But when I click it, nothing happens. And the list of available applications shown by the button in the bottom left corner of the GNOME 3 desktop doesn't offer Thunderbird. With TB 68 installed it did so.

I will ask in an Ubuntu forum.

Before I installed TB 78, there was installed TB 68, and this one didn't have a calendar tab.

Press Ctrl-Alt-C to bring up the Calendar tab.

So I used it to uninstall T 68 and to install TB 78.

If you install both version, make sure to use a different profile for each.

The software app displays a button to start the installed version (now TB 78). But when I click it, nothing happens.

You can always start Thunderbird from the CLI.

You can always start Thunderbird from the CLI.

I tried to do that:

~$ thunderbird No protocol specified Unable to init server: Verbindung ist gescheitert: Connection refused Error: cannot open display: :1

display ": :1" is the TurboVNC session I'm connected to currently. I will ask in the TurboVNC mail list, what's the reason for "Connection refused". But what does "No protocol specified" mean here?

It works now for me: I uninstalled TB 78 and installed TB 68 again. And then I found a solution for TB 68 here:

sudo apt install xul-ext-lightning

Now the calendar tab is back. I just need to do some configs to synchronize it with my calendar provider.