I think your address book may be corrupted.

Export the contents of the address book as an LDIF file. (If you select all address books then they will all be exported, one after the other. The repeated save as dialogs are not a bug)

On the toolbar > Help> Troubleshooting information.

Click on the show profile button. Close Thunderbird. Move all the files names with an MAB extension to your desktop. (I am assuming you exported them all.) Restart Thunderbird. Import the previously exported contacts. using the tools menu in the address book.

Start Thunderbird in safe mode. https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

Does the problem still occur?

Thanks, Matt. This looks helpful. 'm going to wait until I've done an extra backup before trying this. Will let you know if the problem persists thereafter.

Christ1 - I can never predict when the problem occurs, so don't know if your technique would help me. Thanks, though.

Matt --

After mucho techno-intimidation, today I am attempting your fix. Unfortunately there are a few discrepancies that confuse me:

• You wrote to click on the "show profile" button in the Troubleshooting Information dialog box, but I cannot find it there. There is a "Profile Folder" row that has a box called "Show in Finder." Should I assume this is what you mean?
• When I do this I get a folder on my MacBook's Finder called "Profiles," with a bunch of files and folders in it. I see 5 of them have the .mab extension that you recommend moving to my desktop, but you assume I have already exported them all, and I don't know how I wold have done that already.
• I did export the three address books in my Address Book, they are sitting on my Desktop. Now I've quit Thunderbird. I understand that I need to import the three address books, but what do I do with the .mab files?

Thanks, Matt.

Tad

Your mention of "finder" is the first hint that you're using a Mac, not Windows, which will always be the default assumption if nothing is said to indicate otherwise.

There is a "Profile Folder" row that has a box called "Show in Finder." Should I assume this is what you mean?

Thanks, Zenos. What should I now do with the .mab files now sitting on my desktop? throw them back into the Profiles folder? Trash them?

They are there as a fallback if the export-then-reimport doesn't help out. You can at least go back to just where you were.

When .mab files become damaged, we find that exporting to LDIF fixes up the errors, and we can re-create the original address book by importing the LDIF file. If that has all worked for you, you no longer need the .mab files. But do check your imported address books carefully first!

Thanks, Zenos. My address books contain probably close to 1,000 entries, how would I check them? And to complicate matters, I have no idea how many corrupted entries there were before I undertook the LDIF export/import. Have you any suggestions for me on this?

Matt and Zenos --

I tried both your approaches, and the problem persists. Matt's protocol was complicated, perhaps because I'm running an old MacBook with OS 10.6.8, but I thought I'd followed every step eventually. Have you other suggestions? It's scary that I might lose contact information permanently.

Thanks, Tad

Please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information. Press the Copy text to clipboard button and paste the information into your reply.

Application Basics

   Name: Thunderbird
   Version: 45.2.0
   User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:45.0) Gecko/20100101 Thunderbird/45.2.0
   Profile Folder: Show in Finder
   Application Build ID: 20160630070546
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) mail.sover.net:110, plain, passwordCleartext
     OUTGOING: , smtpauth.sover.net:587, plain, passwordCleartext, true
     OUTGOING: , smtpauth.sover.net:587, plain, passwordCleartext, false
     OUTGOING: , smtpauth.sover.net:587, plain, passwordCleartext, false

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

 Crash Reports
   http://crash-stats.mozilla.com/report/index/bp-7aad9206-b5ea-4fa8-b290-5ae542160701 (7/1/16)
   http://crash-stats.mozilla.com/report/index/bp-833e2b1f-5661-4197-a444-55a2e2151107 (11/7/15)
   http://crash-stats.mozilla.com/report/index/bp-ea801734-5159-4167-b11e-e0c302150111 (1/11/15)
   http://crash-stats.mozilla.com/report/index/bp-aba20351-0dcc-4acf-a415-0b77d2141207 (12/6/14)
   http://crash-stats.mozilla.com/report/index/bp-7006ded6-fb4d-4d88-b50f-4c4742141126 (11/26/14)

 Extensions
   Lightning, 4.7.2, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   Signature Switch, 1.6.14, false, {2ab1b709-ba03-4361-abf9-c50b964ff75d}

 Important Modified Preferences

   Name: Value

     accessibility.typeaheadfind.flashBar: 0
     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     extensions.lastAppVersion: 45.2.0
     font.internaluseonly.changed: false
     font.size.variable.x-western: 14
     gfx.blacklist.direct2d: 3
     gfx.blacklist.webgl.msaa: 4
     gfx.crash-guard.glcontext.appVersion: 45.2.0
     gfx.crash-guard.glcontext.deviceID: 0x2a02
     gfx.crash-guard.status.glcontext: 2
     mail.openMessageBehavior.version: 1
     mail.spotlight.enable: true
     mail.spotlight.firstRunDone: true
     mail.spotlight.global_reindex_time: 1399237571
     mailnews.database.global.datastore.id: 3d272ebf-36d2-4d4d-bcf1-6491f2b1cd6
     network.cookie.cookieBehavior: 3
     network.cookie.lifetimePolicy: 2
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1472846532
     places.history.expiration.transient_current_max_pages: 104858
     plugin.importedState: true
     plugin.state.silverlight: 0
     plugins.update.notifyUser: true
     privacy.donottrackheader.enabled: true
     security.disable_button.openCertManager: false
     security.OCSP.disable_button.managecrl: false

 Graphics

     Vendor ID: 0x8086
     Device ID: 0x2a02
     WebGL Renderer: Intel Inc. -- Intel GMA X3100 OpenGL Engine -- 2.0 APPLE-1.6.36
     GPU Accelerated Windows: 0

     AzureCanvasBackend: skia
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: none
     AzureContentBackend: quartz

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.12
     4.12

     NSS
     3.21.1 Basic ECC
     3.21.1 Basic ECC

     NSS Util
     3.21.1
     3.21.1

     NSS SSL
     3.21.1 Basic ECC
     3.21.1 Basic ECC

     NSS S/MIME
     3.21.1 Basic ECC
     3.21.1 Basic ECC

There used to be an add-on (Addressbook in a Tab) which did cause problems for the address book. You do not appear to have that installed. In any case, try to start Thunderbird in safe mode to see if that makes any difference. This has been suggested before. https://support.mozilla.org/en-US/questions/1124608#answer-881401

Ok, I'm in safe mode now. Will pay attention to see if the problem persists.

Thank you, Chris, for your support.