It is an add on. It installs just like any other add on. Go to the add on tab and click extensions. If you already downloaded the correct lightning version for the beta software you are running click the gear icon and select Install from File. You could also just type lightning in the search box on the extension page and see if it finds the correct version and install from there.

Thanks, I'm new to TB and the help files don't say exactly how to do certain things and are very basic. I'll look harder the next time.

No I understand. The help files are all over the place and they address various versions. Some still relevant and some not so much.

You may or may not know that Thunderbird is mostly a volunteer operation since Mozilla stopped developments a couple of years back. Help articles are slowly getting updated. I have better luck using Google and adding the word Thunderbird to the question than using the Thunderbird Help menu.

Feel free to come back and ask questions. That is what this forum is for.

CoreyAF said

Thanks, I'm new to TB and the help files don't say exactly how to do certain things and are very basic. I'll look harder the next time.

Which help file URL are you referring to? https://support.mozilla.org/en-US/kb/installing-addon-thunderbird ?

Where does one find the version of lightning that works with TB 60?

It comes bundled with Thunderbird these days. It seems many people are having issues with the bundled versions. So much so they wrote this article on how to fix it. Maybe it will help. https://support.mozilla.org/en-US/kb/calendar-updates-issues-thunderbird