You may be experiencing the issue described in the following article:

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

Thank you @DavidGG but that isn't the issue and looking at the one other profile shown did not restore things. The worrisome point is that both Thunderbird and Firefox lost data at the same time, which is why I thought it might be a Windows update issue. This is Windows 11 Pro, 24H2, OS build 26199.3194 Plenty of disc and storage space, 64GB memory.

Starting to reconfigure both applications now… :-(

See https://support.mozilla.org/en-US/questions/1489403

@sfhowes Thank you but again that did not help. Bizarrely, having set up my two email accounts on Thunderbird again (IMAP, so all recovered after a while), it again reverted to the initial Set Up screen on startup this morning. This time I have uninstalled Thunderbird, reinstalled and set everything up yet again. I will wait and see if this proves stable. Meanwhile, having checked the "Open previous windows and tabs" in Firefox settings, it opened this morning with the initial screen with no previous tabs open at all, though bookmarks are intact.

Can *anyone* suggest what is happening here? My laptop, also running under Windows 11, is fine but my main desktop machine is the one with all these issues.

Do you have an app like CCleaner or Privazer that resets app settings to their defaults? If it is applied to TB, that has been known in the past to result in what you're seeing.

Addendum: some addresses have been lost from my address book(s), some are still there. Weird...

@sfhowes Yes, I do have CCleaner. That's been installed for about a year but never had any problems before. Worth a shot, I can do without it, so will uninstall and hope that might be the issue. Thanks for the suggestion.