You keep mentioning various Thunderbird versions but none are the current version, which is 52.6.0

there are various versions of Lightning designed to work with various versions of Thunderbird. So it would seem you have not managed to match them. I suggest you start by using the current version of Thunderbird.

Uninstall Thunderbird program via 'Control Panel > 'Programs and Features' and then install the current version. You can get a download for your OS in your prefered language from this link: https://www.mozilla.org/en-US/thunderbird/all/

Thanks Toad-Hall. I now realize the versions of Thunderbird I was using were Beta versions. I also downloaded the correct version of Lightning and both Thunderbird and Lightning are once again purring.

Thanks again, for your help and insight.

I have Thunderbird 59.0.2 (64-bit). says: "Firefox is up to date." When I go to Add-ons and select Lightning, latest version shown is 5.4. But it says this version of Lightning "Works with Thunderbird 52.0 - 52.*". I thought I had installed the Lightning Add-on, it shows up in Tools>Add-ons, but when I go into Options, there is no tab for Lightning, but only for Calendar (which I assume is the default Thunderbird built-in calendar). Is there no working version of Lightning if you are using a version of Thunderbird later than 52.*?

Lightning is the Thunderbird Thunderbird calendar. Firefox has nothing to do with it.

Once you install the current STABLE version of Thunderbird, then go to the Lightning page and find the compatible version to Thunderbird. You may want to uninstall Thunderbird first, all you settings and preferences will remain. The new installation should find them and use them.

For a Current version of Thunderbird:

   https://www.mozilla.org/en-US/thunderbird/all/

Thunderbird and Lightning Compatibility Chart.

    https://developer.mozilla.org/en-US/docs/Mozilla/Calendar/Calendar_Versions

Good Luck!!