You may bet everyone else reading this is wondering the same… Ah! You said in the the title of the post… OK :)

Is the Address Book the only thing that was affected in Thunderbird? Check the following article in case it's relevant to you:

https://support.mozilla.org/kb/recover-user-data-missing-after-thunderbird-update

The article did not provide me with anything helpful to this issue. Are you saying that EVERYONE has lost their address book?

No, I'm saying that something that might explain this could be that Thunderbird is using a different profile for some reason, but then I would expect you to miss more things, not just the Address Book, hence the question I asked.

If you do a search in the c:\users\<yourid>\appdata\roaming\thunderbird\profiles folder, you may find abook.sqlite and history.sqlite. These are the technical filenames of the addresssbook and the collected addressbook. You can import them with tools>import>importfromafile. This sometimes happens, that thunderbird creates a new profile and some content is left behind. No, this does not happen to everyone and is likely a glitch wherein an update was interrupted by another program, such as an antivirus product.