check to see that the "real" msgfilterrules.dat file is not read only.

The msgFilterrules.dat is not Read only.

The next obvious question, based around Thunderbird file access is what is blocking access to that file when Thunderbird tries to download mail that sees another created with the number appended.

Some of the more common things that can cause this occur are;

• Antivirus scanning files "on access" Solution: place an exception in the anti virus to block such activity in the Thunderbird profile folders.
• The location of the profile being somewhere unexpected like the documents folder, so Windows in particular treats the data somewhat differently. Solution: place the Thunderbird profile in a location that is at least not common with document files. Preferable the hidden application data folders that are default.
• The location of the profile being synchronized to the cloud, via one drive, google drive etc. Solution: disable the cloud synchronization for that location.
• The location of the profile is subject to streaming backups. Solution: cease the streaming backups on the profile folder as Thunderbird does not expect anything else to be accessing any of it's files.
• The profile is not located on a local disk and either network latency or a slow server is seeing the files "served" in a less than timely manner. Solution: move the Thunderbird profile to a local disk.

I do not claim that to be an exhaustive list, but just some of the causes I have encountered on these support forums over the years, beyond the file being read only, either through the file system, or network policy or user access policy set on the machine. When checking in the file system to see if the file is "read only" check the security tab to see if you actually have write access to the file.

The profile is on my SSD "D" drive. I did just except the folder from on-demand virus scanning, so I will see how that goes.