Download Lightning 4.7 for your language and install it. https://ftp.mozilla.org/pub/calendar/lightning/candidates/4.7-candidates/build1/win32/

For more information on how to install extensions in Thunderbird see http://xenos-email-notes.simplesite.com/416814616

Why would I want to download lightning separately when it comes bundled since TB 38? Also I already wrote, that the calendar does appear in new empty profiles I create, so it's a configuration issue, not one of the software actually missing. I should also mention that I did indeed try to install lightning via the addons manager but I was informed that it is unavailable for TB 45.

I did manage to fix the issue by hand-editing prefs.js and deleting a line containing "extensions.systemAddonSet" (I think it was this one). After that the calendar did appear in TB

There have been Lightning update problems throughout the entire Thunderbird 38 life cycle, and reinstalling Lightning did fix it in most cases.

christ1 said

There have been Lightning update problems throughout the entire Thunderbird 38 life cycle, and reinstalling Lightning did fix it in most cases.

What do you mean by "re-installing lightning"? When I manually install Lightning the way you have described, I assume, it is installed for the current user only. So I don't re-install it, but I install a second copy alongside the copy already present in the system. So I end up with two copies of lightning, am I right?

Has anybody looked into what caused the problems?

So I end up with two copies of lightning, am I right?

No. Even though Lightning is now bundled with Thunderbird, it is still an extension. It can be uninstalled, re-installed, and updated. You cannot install it twice.

Has anybody looked into what caused the problems?

https://bugzilla.mozilla.org bug 1211291 (esr38, unknown in esr45) – Folders are visible, but messages are not bug 1211358 lightning chrome.manifest not updated in 38.3.0 bug 1249894 Problems updating to 38.6 to do with Lightning

christ1 said

So I end up with two copies of lightning, am I right?

No. Even though Lightning is now bundled with Thunderbird, it is still an extension. It can be uninstalled, re-installed, and updated. You cannot install it twice.

There can be a system wide install (which comes bundled with TB and a second install local to the user. Why not? That's what the solution you proposed seems to result in. Or does the system wide install get uninstalled or replaced when I install the extension? That would require administrator rights.

Each user starting from scratch, i.e. without a pre-existing profile, would receive the prompt to install or omit Lightning. I don't think there is a "system wide" mode for extensions in Thunderbird; they are managed within users' individual profiles.

In theory, if they already have Lightning, it should just appear as usual. Similarly, if they had previously chosen not to have Lightning then it would be silently left uninstalled. That's my experience when beta-testing; we install the candidate version of TB in a clean account and it starts from scratch, offering the choice to have or not have Lightning. We are encouraged to include it to be able to extend testing to cover it. On a later test of updating from the current version to the candidate version, Lightning just reappears, without a prompt, since it was previously installed and operating.

I said "in theory" because one of the problems users have been having is that if they have previously used and then disabled Lightning, the bundling process mistakenly re-enables Lightning for them.