Message filters are stored on disk in text files. One in each account called messagefilter.dat. I suggest you check the relevant files in your profile and monitor of they are say read only, or something is preventing new copies being written. Often multiple files with numbers are also found in this sort of circumstance as Thunderbird's response to a write error is a new file with a number, which it never ever refers to.

Generally this type of error related to file contention caused by antivirus scanning, cloud sync or streaming backups, although remote storage of the profile sometime sees this as well because of network latency.

Thanks, Matt, for your suggestion. I've done a complete search of my desktop computer for messagefilter*.dat, but it found nothing. Not only that, but I have a laptop computer set up in much the same way as the desktop. A search on that also turned up nothing, but with the laptop changes in the message filters *are* saved! Could the message filter file have a different name than messagefilter*.dat?

P.S. I also tried running Thunderbird in troubleshooting mode and made some changes in the message filters. When I exited and reopened Thunderbird in normal mode, I found that the changes had not been saved, just as before.

msgfilterrules.dat Otherwise, Matt is right on.

Thanks David. I've found the msgfilterrules.dat file, but it's not being written to. I need to investigate further but unfortunately don't have time right now. I may be back!

Unfortunately, I am back! I'm confused: The file MsgFilterRules.dat is stored in a particular folder (not the default). If I make changes in the file with a text editor, those changes are reflected in the list Tools > Message Filters in Thunderbird when I open it. However if I then click "Edit" and make a change in or delete one of the filters, the change is gone if I close and then reopen Thunderbird. Nor has the MsgFilterRules.dat file been changed. Why doesn't Thunderbird save the changes?

Unfortunately, I am back! I'm confused: The file MsgFilterRules.dat is stored in a particular folder (not the default).

It will be stored in the accounts "local directory" that is shown in the server settings for the account. I am not aware of an option to redirect it to somewhere other than that.