I don't understand. You have to select to install Lightning. I guess you missed the prompt, or you have experienced a bug whereby a disabled Lightning is incorrectly re-enabled when upgrading Thunderbird. It certainly isn't intentional that Lightning would be installed or activated without your explicit approval.

Go to Tools|Add-ons and remove Lightning.

Tools|Options provides settings for how Thunderbird updates. Many users would expect add-ons to update automatically, especially when a Thunderbird update would deactivate installed add-ons. But I agree with you that add-ons should never install themselves uninvited.

This is what happened: After a restart, thunderbird interface has changed. To the right there is a calendar pane. On the bottom a yellow alert pops up saying something like "lightning has been installed, you can disable it". I click disable it, and then start to investigate what it could be. I find it in the add-ons section, now listed as installed but deactivated. I click to remove it

I believe this happened alongside with the 38.1.0 update and I believe I have never installed or activated any add-ons. The alert text gave the impression it was something forced on me with an fine printed opt-out in hope of people not noticing. But I can be wrong of course.

Anyway, I have now switched to not accept automatic updates and will watch it more carefully in the future.

"After a restart"? That shouldn't have happened. We offer Lightning as part of a new installation, but if you're upgrading, it should respect your existing settings. As I said, there appears at present to be a bug whereby an inactive Lightning incorrectly becomes re-enabled.

The bundling of Lightning is a response to requests from many users to incorporate it as standard. Lightning is not actually embedded into Thunderbird, but is offered automatically to new users. It's not clear why you as an existing user would have Lightning appear from nowhere, unless indeed you have installed and deactivated it in the past.

Restart as in Computer restart. Like I said, I can't remember installing any add-ons at all, but I can't swear on it. I have had Thunderbird for a long time.

I do understand why users would want the calender and other add-ons to automatically update. I was just surprised because I didn't know I had it, and it certainly wasn't activated before.

Inside my application there is a active_update.xml file and if I browse it, it seems I was updated to 38.1.0 last friday (installdate=1437123025327) and that it was a pending install. The computer is "always on" but was restarted yesterday. After that, launching thunderbird resulted in what I described above. No install dialogue, it launched normally but into the new interface.

And now it did it again...

Prompted me for an upgrade to 38.2.0 and back is the Lightning add on I carefully removed last time. Pretty annoying.

Is it installed or does it ask you if you want to install?

It does not ask. What happens is this: Thunderbird says there is a new version. I agree to download it. After new version is installed, a small yellow bar highlights the bottom of thunderbird and says something like "we have installed Lightning for you. Do you want to remove it?" I choose remove it and I need to restart once more.

The add on is not removed but deactivated. I go into addons and remove it and restart again.

It does not ask. What happens is this: Thunderbird says there is a new version. I agree to download it. After new version is installed, a small yellow bar highlights the bottom of thunderbird and says something like "we have installed Lightning for you. Do you want to remove it?" I choose remove it and I need to restart once more.

The add on is not removed but deactivated. I go into addons and remove it and restart again.

The new status quo is that when you download Thunderbird, you also get Lightning. That is intentional and I suppose means that the answer to the question in your topic is "you can't" . The developers, it would appear, are moving towards the position where Lightning is a fundamental component of Thunderbird, similar to the way when you download Thunderbird you also get its Address Book.

I think that when the idea of Lightning being integrated into Thunderbird was floated, many of us assumed it would become built in, and not presented as an addon. So long as it is implemented as an addon, you can remove it if you so wish. But it may yet become a permanent feature, with an option to disable it.

For the present, we have a bug whereby if Lightning has been installed in the past, it is incorrectly re-enabled when upgrading. Like you, I would very much like to see this bug fixed.

Folks. Bug 1182629 applies here I think Zenos, Gnospen are you aware of other instances that can be linked to the Bug?