ok you have me confused.... what does firefox installing and re installing have to do with Thunderbird and the lightning add-on.

Have you looked on the add-on entry in the application menu to see if there is a problem with Lightning, perhaps it did not update when Thunderbird did like should have.

I solved the problem by going back to 3 month old versions, erasing chrome files and setting Thunderbird v.24 to not update. Enough is enough time wasted on losing data. Thanks.

I got a book for a reply but few answers. Matt said

Have you looked on the add-on entry in the application menu to see if there is a problem with Lightning, perhaps it did not update when Thunderbird did like should have.

Perhaps the issue is you did not press F10 to see the menu so you could select Tools and then Add-ons or you did not use the application menu to select add-ons and look at the installation of Lightning.

Is it installed? Is it enabled? What version? Can it find updates?

Firefox is and was set up to update automatically AND to warn me if add-ons need to be updated manually. I got no such warning. The Lightning calendar just disappeared a week ago or so, so I looked to see if my Thunderbird needed manual updating, but it had updated fine, as it was set. I disabled all add-ons and got fresh versions of both, uninstalled Thunderbird after making a backup, and installed the fresh copy. Now I wish I had never done that, because that is how I lost my address book. -

The part of the explanation below the line is extra stuff in case it applies, and like this, is not required reading..... More info is better than not enough. Been having a rough January, losing control of my Facebook, Google+, Youtube and cellphone accounts and now this. I just cannot believe this further loss of my personal data is a coincidence.

This is just information gathering. on your start menu type %cmd% and press enter. in the command prompt windows that opens

type cd %appdata%\thunderbird and press enter.
type notepad profiles.ini

Copy and paste the contents of the file that opens in notepad into a message here.

type CD profiles and press enter
type dir  > notes,txt and press enter
type notepad notes.txt and press enter

copy and paste that from notepad to the message here

type exit and press enter to dismiss the command prompt.