Have you tried removing it and adding it again? I think you should.

Matt, Thanks for the reply. I re-installed lightening and the calendar re-appeared, with all my appointments (thank goodness!) It begs a question about the stability of FF updates and how much is checked before issuing,

It is a bug that has plagued recent releases, The developers have thought they had is pinned down in the last few minor releases, but it has a away of reappearing with a slight variation. A part of the issue is only a very small number if updates are involved and therefore the bug is difficult to reproduce.

With such a simple manual solution to repair it is of concern, but not a show stopper for releases. If the calendar data was actually lost, then update would probably have been halted.

https://bugzilla.mozilla.org/show_bug.cgi?id=1249894

You can follow the bug at the URL if your interested.

I have been having this problem over and over since last year, and I am not the only one. I have posted about it several times and I have seen a number of other posts from people with the same problem.

Unfortunately, reinstalling Lightning DOES NOT WORK for me and many others. After you do the install and click the "Restart Now" on the add-in, it restarts but is not installed and still has the "Restart Now" link. Even the "Undo" does not work.

I, and others, would appreciate a fox/solution for this.

Thanks!

Looking thru the posts I see several (including one I posted but there have been no answers) that are related to this Lightning problem. I think they are all related to the same bug.

More info:

Several minutes after I installed Lightning, I got a tab asking for permission to install Lightning. It then shows Lightning 4.0.5.2 as installed but the "wait" circle is going around on the Add-Ons Manger tab but there are no options in the menus to show calendar or tasks tabs.

One other funny thing, there is no "Help" menu after "Tools" menu.

FEUM49 said

I have been having this problem over and over since last year, and I am not the only one. I have posted about it several times and I have seen a number of other posts from people with the same problem. Unfortunately, reinstalling Lightning DOES NOT WORK for me and many others. After you do the install and click the "Restart Now" on the add-in, it restarts but is not installed and still has the "Restart Now" link. Even the "Undo" does not work. I, and others, would appreciate a fox/solution for this. Thanks!

Dummy88 said

Just had the update to Thunderbird 38.5.0. Lightening was working fine before, but now there is no calendar tab or option to display it. Lightening is still in the Addons but disabled as "not compatable". I rely on the calendar continuously so now I am desperate. How can I get it back? Is there a way to roll back the update? (It appears that this is a common problem with this update) Thanks for help

Dummy88 said

Matt, Thanks for the reply. I re-installed lightening and the calendar re-appeared, with all my appointments (thank goodness!) It begs a question about the stability of FF updates and how much is checked before issuing,

Happened again! Lightening calandar "incompatible and disabled". Applied same fix, remove and reinstal. This is becoming a real pain. I have stopped recommending this app, Please get it fixed, or stop issuing updates, or provide a method of returning to the previous version of Thunderbird.