"Go to the Advanced section of Thunderbird OptionsSettings and click on Config Editor...
   Copy or start typing extensions.installedDistroAddon.{e2fda1a4-762b-4020-b5ad-a41df1933103} into the search box
   When extensions.installedDistroAddon... is listed in the preferences list, right-click on it and choose Reset from the context menu - the preference now isn't bold anymore and its value is empty
   Close the editor, leave the Thunderbird options, open Thunderbird's add-ons manager and switch to the list of installed extensions
   If Lightning is listed there, click on Remove to the right of the list item
   Restart Thunderbird"

I am trying to fix the loss of my calendar and am using the following off your site.Lightning disappears after a Thunderbird update (release and beta versions) This may happen if the update of the add-on is stuck. Don't worry about your calendar data, it is left untouched and will be available again if you follow these steps to restore Lightning: "Go to the Advanced section of Thunderbird OptionsSettings and click on Config Editor... Copy or start typing extensions.installedDistroAddon.{e2fda1a4-762b-4020-b5ad-a41df1933103} into the search box When extensions.installedDistroAddon... is listed in the preferences list, right-click on it and choose Reset from the context menu - the preference now isn't bold anymore and its value is empty Close the editor, leave the Thunderbird options, open Thunderbird's add-ons manager and switch to the list of installed extensions If Lightning is listed there, click on Remove to the right of the list item Restart Thunderbird" After Thunderbird has been relaunched, Lightning will be back and you will have access to your calendar data. When I key in the info required from above the extension is not listed. Does that mean that since it is not there that I can continue with the directions and the calendar will appear?