Thunderbird is downloading messages in code and I am also getting an error message

My wild guess is you have an antivirus issue. The image you posted shows nstmp files. These occur when a compact process is aborted. You have garbled mail showing. A sign that updates to the folders index (MSF file) are failing. Finally you are getting an error message about disk space (which I assume is spurious) All of those things are caused by file contention and the largest single cause in Windows systems is anti virus scanners. Create an exception in whatever you are using to exempt the Thunderbird profile folder from scanning. After anti virus scanners are streaming backups and cloud synced storage locations, followed further back by network storage for the profile folder.