Are you an administrator on your computer or a restricted use user? Do you do any better if you start Thunderbird by right clicking and selecting run as administrator from the menu?

Yes, my user account is an administrator account. I tried restarting the computer and launched TB to run as administrator but no change in behavior of Lightning. When loaded from a file, the AOM changes to remove the bundled Lightning and shows a installation beginning (screen 3). But the progress bar never moves and then a second restart prompt appears at top (screen 4). Restarting in any way just returns to the original condition (screen 1). The error console continues to generate error messages (screens 5) that seem related to the install attempt. But I don't know how to interpret or correct them.

Thank you.

do you have local calendars that have saved data, or are your calendars remote. like Google or own cloud. I am trying to decide if we can just delete everything calendar and reinstall or if their are local calendars that must be saved.

No local calendars.

go to the Thunderbird help menu, select troubleshooting information click the show folder button. Close Thunderbird in Explorer delete the folder calendar-data Try again in Thunderbird

No change. I do see the calendar-data folder is not being recreated. I should say that once during a week or so of trying to get Lighting to enable/load the calendar tab did appear. I'm guessing that is when the calendar-data folder was created. I don't know what I did or why that happened but then I installed Provider to sync the calendar and Lighting went away. I've tried disabling Provider without any effect. I've tried removing it but it reappears in disabled mode; don't understand why it doesn't remove entirely. The error console log may provide a clue. Latest attached. Seems to indicate an 'addons.xpi error involving the profile folder 'ziohp3ew.default' from earlier 'troubleshooting information' button. Continued thanks.

go to the extensions folder shown in the error console in windows explorer. Delete the folder with the guid listed. (string of numbers) It looks to me like it is trying to remove something so lets try and do it first.

Success! Deleting that folder did the trick. Lighting is enabled, working and syncing through Provider. Thank you for your help.