See https://support.mozilla.org/en-US/questions/1074936#answer-768866

Tried several solutions (including Crist1's helpful idea) and Lightning 4.0.2 still does not install properly in Thunderbird 38.2. I tried the earlier EN build identified in the response. I waited for a few days and got a later version of 4.0.2. I uninstalled the Google provider, reinstalled Thunderbird.

The Lightning download installs, with a message "Lightning will be installed after you restart Thunderbird." But it does not install.

I have a fairly vanilla Windows 7 professional machine, and only Adblock extension at the moment.

This looks to me like a Lightning - ver 38.2 compatibility that likely affects a lot of people.

I waited for a few days and got a later version of 4.0.2.

I don't understand what that means.

what "later version" means. Version number was the same, but in the download directory it had a later date. I was speculating that this was a problem for many users and that it would get corrected.

Up until 38.2 I had used Lightning for several years without problems, although a Lightning up date was frequently required.

I am reasonably experienced with software and can usually resolve these problems on my own. Had a few issue with webroot anti-virus software interfering with some software-internet operations but have ruled that out here.

From your reply I'm not sure whether you tried the candidate build or not. The difference to the official build is that it doesn't contain all the localizations. The official Lightning build dumps a ton of files into the extensions directory, so it may well be related to anti-virus. Also, you may need to delete the 'staged' directory underneath 'extensions' within the profile before. The problem doesn't appear to be widespread, otherwise this forum would be full of complaints.

Thank you christ1 for staying with this problem. Other users: the problem was resolved with the last response from Christ1. I deleted the 'staged' directory underneath extension. Deleted all other extensions. Then installed the EN language candidate build.

christ1 is also right this is probably not widespread. Upgraded to 32.2 and Lightning 4.0.2 on another Windows 7 computer (same antivirus) and no problems whatever.

I had this problem too. I closed Thunderbird, deleted the 'staged' directory in the profile, then restarted it and reinstalled from the manual en-us download found here:

https://ftp.mozilla.org/pub/calendar/lightning/candidates/4.0.3-candidates/build1/win32/

I am currently on Firefox 41.0.2, but it has been a problem for months and with earlier versions. Someone else mentioned it started with version 38 and Lightning 4.0 and that sounds right to me.

I don't know why Lightning suddenly uninstalled itself from Thunderbird, and wouldn't reinstall after repeated restarts ("Lightning will be installed after you restart Thunderbird.") This is what finally fixed it on my Windows 7 Pro laptop:

• Download latest Thunderbird Setup (.exe) and Lightning (.xpi) to desktop.
• Control Panel > Programs and Features > Uninstall Mozilla Thunderbird. Note: This does NOT discard email, accounts, or filters.
• Navigate to C:\Users\YOU\AppData\Roaming\Thunderbird\Profiles\ikmlr38k.default\extensions\ (this "profiles" folder might have a different name).
• Delete Lightning folder {e2fda1a4-762b-4020-b5ad-a41df1933103}
• Delete "Staged" folder (if it exists).
• RESTART computer (fix didn't work without this).
• Click Thunderbird Setup (.exe) to reinstall - use defaults.
• In Thunderbird, Check Add-Ons, click Install Add-On from File, find and click the Lightning (.xpi) installation file. Let it finish.
• Restart Thunderbird when it says to do that.
• That should do it.
• If not, try once more starting at step 2 above.

It only takes a few minutes to do this, but it took me hours to find the fix. I don't think the problem is related to Thunderbird or Lightning versions - this fix worked with the latest downloads as of 2015 Nov 12, and it worked last June too, doubtless with different versions.

Update: This happened again with Lightning 4.0.4 and Thunderbird 38.4 The Lightning add-in appears in the Extensions directory with prompt to restart Thunderbird to complete installation, but nothing happens.

minnesotadon's helpful solution did not work until I downloaded the EN build rather than the regular distribution. (Another 2 hours wasted on this recurring problem) (The likely reason for this strategy is that Webroot or some other process is blocking the automatic extensions installation. Therefore you need to download the xpi file. Manually install it using the "gearwheel" option in extensions tab.)

https://ftp.mozilla.org/pub/calendar/lightning/candidates/4.0.4-candidates/build1/win32/

Thank you TJM!

I wonder if Sunbird (standalone calendar) should be brought back, for those of us who would prefer the calendar to be a separate app anyway.