Create a new profile in Thunderbird for testing

Then quit Thunderbird. Then copy your abook.mab to the new profile. Then run Thunderbird with the new profile and see if the Address Book will open.

Note: abook.mab is only for the "Personal Address Book" which is standard in Thunderbird. See Moving address books between profiles for more information on this.

If you right-click abook.mab, Properties, is it Read Only? It would be if you copied it from a CD or DVD backup. Exit TB if you need to uncheck the Read Only property.

To sfhowes: Yes, I checked it and it was not Read Only

To Bruce Johnson: I set up a new profile and copied abook.mab. It worked there fine. But if I copied the entire profile of the original into the new profile, it was the same thing: the address book will not open in the new profile either. So it seems that the address book itself is ok, but something else prevents it from opening in the original profile. I am not sure what else could influence it, or how to proceed.

To both of you, thank you for your suggestions and welcome any other ideas.

Now that the address book opens fine in the "test" Thunderbird Profile, make a change to it in that profile. Add a contact, edit a contact, anything will make the test profile re-save the .mab file.

Quit Thunderbird, then Copy (not move), that .mab file to your original Thunderbird Profile. I say copy, not move, so the address book file will also still exist in the test profile.

Then start Thunderbird with your normal profile and see if the Address Book will load.

If it still will not load, then consider using the new profile you created from now on. See my instructions for Create a clean new profile in Thunderbird and import your stuff from the old profile.

First thing to try is to run TB in safe mode (hold Shift when you launch TB). Although I can't think of an add-on that would cause this particular problem, there are other consequences of running in safe mode.

http://kb.mozillazine.org/Standard_diagnostic_-_Thunderbird

Since you have already know how to create another profile, it may be useful to selectively copy the contents of the old profile as suggested in the above article, assuming the problem isn't due to an add-on.

Yes, try running Thunderbird with add-ons disabled.

If that doesn't produce good results, then follow the instructions I gave for moving your stuff to the new profile.

Sometimes Thunderbird profiles just get messed up, so starting fresh with a new profile fixes things.

Thank you both for your suggestions.

Disabling my add-ons did not produce the hoped-for results. Before I could try the transfer suggested by Bruce, there was a Windows update on Tuesday, January 10th, 2017.

After this update, the Address Book on the unchanged Profile became accessible again, reappeared and is working normally.

It is perhaps relevant to note that I have not changed anything in the Profile. I have no idea how the Windows update could have affected Thunderbird, but this may be a relevant piece of information.

Again, I would like to thank you for your valuable and reassuring advice. Much appreciated.

I don't know how the "Cumulative Update" could affect Thunderbird, but it is possible that the "Windows Malicious Software Removal Tool" also downloaded and ran. It usually does at the same time as the other update.

The "Windows Malicious Software Removal Tool" may have found something bad and removed it.

To be safe, I strongly recommend that you check the security on your computer. Make sure the antivirus and firewalls are working and up-to-date.

Run scans with additional anti-spyware and anti-malware scanners. I recommend using the free versions of both Spybot Search & Destroy and Malwarebytes Anti-Malware.

Checking the security is always a good advice. In my case, I am running Malwarebytes (full version) and Microsoft Security, monitoring everything I do. My firewalls are up and working. Daily scans alert me of anything that has been quarantined. There is no indication that anything was found and removed during the Windows update.

It is possible that the Windows update and the reappearance of the Address Book are just correlated, but not causally-related events. In that case, the mystery of why the Address Book became inaccessible and then 'cured' itself remains unsolved.

Just another piece of information that may interest you Bruce.

I told you earlier that the problem disappeared after a Windows update. Well, after a while it reappeared again, and so I decided to follow your instructions for a new, clean profile.

The process was completed successfully, but unfortunately, the erratic behaviour of the Address Book continued.

During the process and after the installation I got a message about Chrome running a script in the background. I figured that this has to do with my Thunderbird also connecting to the my gmail account. So I decided to remove the gmail account from Thunderbird since I am not using gmail much anyway. Lo and behold, the problem with the Address Book disappeared!

The way I figure, Chrome was running some script related to gmail that got hold of my address book and this is why it became inaccessible. With the gmail account removed from Thunderbird, the Chrome script was no longer running.

My Address Book is now working fine.

Once again, thank you for your help and the instructions to create a clean profile. The process helped me find the source of the problem.

Best regards Charles