Open the add-on tab. Click Extensions. Type Lightning in the search box. Install it.

When you download the file you go to the icon that looks like a gear and select Install for File.

I did apparently get it downloaded but when I install it, I'm asked to restart Thunderbird to "activate" Lightning. I've restarted twice and nothing. When I look at my list of extensions, I see them all but Lightning keeps telling me to "Restart Thunderbird".

If you can remove it do that and install with the search on the add-on page.

Now I no longer see Lightning on my list of Extensions. I can't find it again to download by using the Search box. The .xpi file I have doesn't see to work because when I click on it all I get is a message that Adobe Acrobat Reader can't open it. Where is Lightning?

Problem resolved itself. Not sure what happened but I went to the Lightning homepage , download the extension again, restarted Thunderbird and it works as advertised.