open the profile folder in a file manager.

Check for the existence of a prefs.js file. More importantly check for a load of them with numbers in them. IF they exist is is a symptom of Thunderbird failing to be able to save it's settings on shut down. The usual problem is an antivirus product poking around and getting all excited when it finds a .JS file (JavaScript). The solution is exclude the antivirus product form the profile folder.

There are other causes, just the antivirus is most common. Others include the disk space occupied by the prefs.js file being damaged so unable to be written. Chkdsk is the checker of choice on Windows. The power to the computer going out before the disk write on shutdown is complete can see setting not saved, but not the numbered files. This is generally folk turning off a power strip on their way out or some sort of all lights and power off switch that is flicked when the door is locked for the night.

Thanks for your response. However I don't think that is the issue there a plenty of prefs.js files however the last one that shows an update is March of 2022. This problem only began last December. I also noticed that is did create new MFS files are there a several there corresponding to the date I created them. I am looking in AppData\Roaming\Thunderbird\Profiles should I be looking somewhere else. Thanks