I've been using Thunderbird without a hitch for years. All of a sudden, when I started Thunderbird this week, I got the message "Unable to load address book file abook-4.mab. It may be read-only, or locked by another application. Please try again later." The file is definitely not read-only, and I cannot see that it is being used by another application. I have tried all of the following, to no avail: 1. Copy the address book file (abook-4.mab) from a backup that definitely was working. 2. Delete all the address books (thereby forcing Thunderbird to create new ones). 3. Delete the files directoryTree.json, folderTree.json, localstore.rdf (forcing Thunderbird to recreate these files). 4. Run Thunderbird in safe mode. The closest I can get is in safe mode. Then I am able to create a new address book and add new contacts to it, but when I restart I get the error "Unable to load address book file abook.mab etc." and no address books or contacts are visible. There are, however, a whole lot of .mab.bak files in the profile folder. When I create a new profile and open Thunderbird in that profile everything seems fine. So it seems to be a problem with the original profile. However, I have years' worth of work in it, so it's not an option to abandon it. Can anyone give me an idea of what file/setting in a profile would cause this problem with the address books? It's driving me crazy! Thank you in advance! Regards Bruce