I had a call today from a client who was having difficulty previewing files in Thunderbird. When he clicked on the PDF it would open a blank screen in Thunderbird. If I changed the default to Firefox it just would not open. I also tried to delete the line in settings, which did not help. I also re-downloaded Firefox and installed a fresh copy. Still no luck!
What I learned today was that you can run a repair just on the specific folder that is having issues. In this case, it was the inbox. Right-clicking on the inbox and running a repair fixed the issue. After all these years I cannot believe that I did not know about this repair option.