> Past threads here suggest reinstalling the Lightning extension but that doesn't make sense, it's already integrated.

The fact that it is integrated (packaged) doesn't make it immune to problems - in the end it is still an addon, with all the potential issues that brings. :)

We are really overdue to have a KB article to help people with calendar issues like this. Perhaps you would be willing to write one up?

After much testing by a colleague, as it turns out Lightning is not part of the core functionality - the extension is bundled with Thunderbird and acts as any other regular extension would, meaning it can be removed or added back etc.

I was convinced its code was now part of Thundebird and not separate.

The fact it's still an extension on its own (even if it's included at install time) means it can't be "reactivated" after removal (or disappearance), it has to be downloaded again, installed and activated as any other extension. It's a subtle but important difference.

@wayne, surething, I've started reading again all the KB guidelines, I'll put something together.

Language check - I mean the addon is integrated with each release process, so you are correct it is bundled. And there should be no need to do a separate install or download when updating, unless you change release channels - beta to release, or beta to nightly, etc.

But there seem to be circumstances in which this process breaks.

Highly recommend you consult with the people in #calendar on irc.

I missed the phrase about removal - yes, if someone actually removes it then it must be reinstalled. If they only disable the addon, there should be no need to reinstall

I suspect when Thunderbird is another locale than US-en Ninite resets everything and removes extensions. This remains to be tested but I wanted to keep a trail here in case others come across this.

see https://bugzilla.mozilla.org/show_bug.cgi?id=1215828 for general update issues

Short answer: re-install from the ...\distribution\extensions\{e2fda1a4-762b-4020-b5ad-a41df1933103}.xpi file distributed with your version of Thunderbird.

I had my calendar disappear (including menu items Events and Tasks and View->Calendar) in Thunderbird 60 beta after I received a prompt that said it was incompatible. After doing some research on Mozilla Support and seeing numerous suggestions to reinstall the Lightning add-on (which does not work for beta), I realized that the easiest way to reinstall the add-on was to use the ...\distribution\extensions\{e2fda1a4-762b-4020-b5ad-a41df1933103}.xpi file that is distributed with your version of Thunderbird. To do this re-install of Calendar extension, drop down the menu for the gear icon Tools\Add-ons, select Install Add-on From File... and in the file selection dialog browse to "C:\Program Files\Mozilla Thunderbird" or "C:\Program Files (x86]\Mozilla Thunderbird" or whatever the equivalent is on your computer, and open the distribution folder and then the extensions folder.

Thunderbird on my Linux system just upgraded itself to 52.9.0 and auto-removed Lightning as incompatible with this Thunderbird version, as described in other posts in this thread. First off, this is horrible behavior! How do you expect Thunderbird to be used in a regular office environment when it does this? Not only hacker-geeks use Thunderbird.

My problem is now to get the Lightning addon back. In Thunderbird I clicked tools > add-ons, and get the Add on Manager page. There is nothing on this page (that I can find) that lets me find add-ons. I get 6 "Featured Addons", no search box for others. There is a link way at the bottom "Browse all Add-ons", but when I click that I get a SeaMonkey page asking if I want to make SeaMonkey the default for Browser, Email etc. Nothing about Thunderbird add-ons.

How do I find and re-add the Lightning add-on to Thunderbird?

To Follow up - I noticed that when I hovered over the "Browse all Add-ons" link it shows addons.thunderbird.net, even though clicking on that took me to the SeaMonkey page. So, I entered the addon.tunderbird.net address into a broswer and was able to download lightning-5.4-sm+tb-linux.xpi.

After some more web searching I found a link that says to add an add-on go to the Extensions selection on the Add-ons page, not the 'Get Add-ons'. Another point of confusion. Is Lightning an Add-on or an Extension? Anyway, the 'Extensions' page shows Lightning as 'Disabled' with no apparent way to enable it. Since I had just downloaded the xpi file I clicked on the gear icon beside the search box and selected 'Install Add-on from file', and selected the file I had just downloaded. Nothing happened. I restarted Thunderbird, went back to the Extensions page and removed Lightning. I then did the 'Install Add-on from file' fagain, and that worked. Lightning is back. All in all, this is a bad and needlessly frustrating process. Good luck to a non-technical user to figure it out. My 2-cents worth.