After some more searching, I found a thread that suggested Anti-Virus might be the culprit. (I'm running Avast.) I tried the instructions in one of the responses: https://support.mozilla.org/en-US/questions/1097411#answer-814160

It did not help. Installing Lightning from the add-ons page and restarting Thunderbird did create a new "staged" directory in: %USERPROFILE%\AppData\Roaming\Thunderbird\Profiles\ftgo62r3.default\extensions\staged\ and the "staged" directory contains a folder called {e2fda1a4-762b-4020-b5ad-a41df1933103} but Lightning does not install, even with Anti-Virus disabled.

You need to follow all the steps, not just turn off anti-virus.

christ1 said

You need to follow all the steps, not just turn off anti-virus.

I did follow all the steps. That should be obvious from reading my description of the results.

I did follow all the steps.

Good. You may try again.

That should be obvious from reading my description of the results.

It isn't.

I am happy to try again.

I disabled Avast Anti-Virus and quit Thunderbird.

There is no %USERPROFILE%\AppData\Roaming\Thunderbird\Profiles\ftgo62r3.default\extensions\{e2fda1a4-762b-4020-b5ad-a41df1933103} directory, but there is a %USERPROFILE%\AppData\Roaming\Thunderbird\Profiles\ftgo62r3.default\extensions\staged\ directory.

I deleted the "staged" directory and then launched Thunderbird.

In Thunderbird, I navigated to the Extensions left menu item. Lightning was not listed.

I navigated to the Add-Ons Manager and selected "Get Add-ons" from the left menu. I clicked on Lightning, then clicked the "Add to Thunderbird" button, then the "Install Now" button. There was no visible response to clicking the "Install Now" button, other than the dialog disappearing.

I selected Extensions from the left menu. Lightning 4.0.5.2 was listed, and it said "Lightning will be installed after you restart Thunderbird." I clicked the "Restart now" link, which restarted Thunderbird.

After Thunderbird restarted, Lightning still was not installed. Lightning was still listed under Extensions, and it still says "Lightning will be installed after you restart Thunderbird."

A new "staged" directory has appeared in %USERPROFILE%\AppData\Roaming\Thunderbird\Profiles\ftgo62r3.default\extensions\staged\ but there is still no %USERPROFILE%\AppData\Roaming\Thunderbird\Profiles\ftgo62r3.default\extensions\{e2fda1a4-762b-4020-b5ad-a41df1933103} directory.

I then verified that Avast Anti-Virus was disabled, and then reenabled it.

Got you. Please download the Lightning .xpi from AMO. https://addons.mozilla.org/en-US/thunderbird/addon/lightning/

Repeat the process. When you get to the step installing Lightning, install it from the downloaded .xpi.

Further instructions here - see 'A slightly less ideal case'. http://xenos-email-notes.simplesite.com/416814616

Thank you, but installing from XPI did not help, either.

I disabled AV, quit Thunderbird, and downloaded lightning-4.0.5.2-sm+tb-windows.xpi from https://addons.mozilla.org/en-US/thunderbird/addon/lightning/

I deleted the "staged" directory just like before.

Then I launched Thunderbird. I used the gear icon at the top right of the Add-Ons Manager page to install Lightning from the XPI file. I saw a progress bar which claimed Lightning was being installed, and then a link that said "Lightning will be installed after you restart Thunderbird." I clicked the "Restart now" link, which restarted Thunderbird.

As before, Lightning was not installed, but a new "staged" directory appeared in the same location described above. The "Lightning will be installed after you restart Thunderbird" link is still there.

To determine if it is not bug 1211160 please download a fresh copy of Thunderbird from getthunderbird.com and install it.

To take the third party issue a step further. Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. for instructions on stating your operating system in safe move see Windows 7

This will allow you to do the install with the absolute minimum of third party processes that might interfere. I don't know if it will fix the issue, but at least it gives a better chance.

If none of that works, I think it is also probably time for a Bug to be raised on this.

I downloaded Thunderbird 38.5.1 from http://getthunderbird.com. Then I rebooted into "safe mode with networking" and installed Thunderbird. I did not uninstall Thunderbird or anything else first: I just double-clicked on the installer.

It worked! I didn't need to reinstall Lightning again or anything else.

It did take awhile to sync with GCal after I rebooted again (into normal mode, not safe mode), but it seems to be sync'ing now.