My guess is something is preventing thunderbird from updating prefs.js file. It may be an antivirus or firewall or other app that intercepts. Another possibility is that prefs.js is read-only. I suggest exiting thunderbird and checking. I wouldn't be surprised if you found multiple copies of the file.

Thanks David that's helpful, I've looked online to try find where the prefs.js file should be located, and think it should be in c:/users/username/appdata/roaming/thunderbird/profiles In that directory I have 2 folders. The first 6zpn03ff.default was last modified in july 2019, so guessing this is an old version. That folder does contain a single prefs.js file again last edited in july 2019 The second folder wrwep6a5.default doesn't have a prefs.js file, but does contain 25 files named prefs-1.js to prefs-25.js The first 24 of these are dated from 2020, prefs-25.js is dated 3/6/23 and does look to contain my latest account information - that date is probably the last time I changed my account settings. The file isn't set to read only. Any suggestions?

Go to the troubleshooting information on the help menu. It has a handy link on the first page to open your profile folder. No guessing then which of those profile is the right one.

Now to these multiple prefs files. Start with an exception in your antivirus product for the Thunderbird profile folder (that is why using the troubleshooting information to get the right one is so important) Thunderbird does not share well. When a file is not available for any reason, or is locked by something like antivirus, cloud synchronization or even streaming backups or a slow network when the file is on a NAS device Thunderbird will just write a new one and add a number, then ignore the file ever more as it has the wrong name. A dumb action, but one that goes back all the way to Netscape.

I would suggest you close Thunderbird, delete all of the numbered versions of the prefs file and then make your update. Watch to see if your exception in the antivirus has stopped the appearance of a numbered version and the apparent loss of your changes.

Thanks Matt, I've no idea whether this issue is related to the other issue I've posted on about the privacy tab, but I think the privacy tabs started appearing in the middle of last year, about the same time the highest numbered prefs-25.js file was created.

I've checked the troubleshooting info and the profile folder is the one containing 25 numbered prefs files.

I've tried turning off my windows defender firewall, mcafee virus software and mcafee advanced firewall, but none of these have fixed it. I see a number of files in the profile folder with today's date, so it doesn't look like anything is blocking it from writing to that folder.

I then tried removing all the 25 numbered files and storing elsewhere, but that still doesn't work.

Then I thought I'd try putting the most recent file prefs-25.js back to the folder and renaming it to prefs.js but it won't let me, it says there's already a file with that name in that location. But this doesn't make any sense as file explorer is set to show all hidden files, folders and drives.

Did you get this fixed? prefs.js is a mandatory file for thunderbird. be sure thunderbird is not running when you delete and rename.

No still not resolved. I took the highest numbered prefs file, prefs-25.js and renamed to prefs.js while in another folder. Then tried copying into the profile folder. File explorer gave warning saying a file with that name already exists in the folder, even though it is completely invisible despite setting file explorer to show hidden files (which seems completely bizarre to me!). So I told file explorer to copy anyway overwrting the invisible file. The new one copied in is now invisible. Restarting Thunderbird nothing appears to have changed, I have the same issue being unable to save any account changes.

Something, as Matt indicated, is interfering. Is the prefs continuing with -1, -2 copies? Can you do a screenshot of the profile folder?

I attach a screen shot of the folder, showing there's no prefs.js file or any numbered variants appearing. This is the folder listed in the troubleshooting info, and I've shown on the screenshot that the file explorer settings are turned on for hidden files. Second screenshot shows files dated from today, so I can see this is definitely the correct folder and thunderbird is writing to it ok, just not a visible prefs.js file

Well, I admit I've never seen such, and there are folders missing and the address book is also hidden. Puzzling to me. Thanks for sharing.